Remove check for user being applicable codeowner

I mistakenly believed this to be desired behavior, and as part of
cleanup work ahead of a codeowner's project I "fixed" this. I had
thought that we wanted codeowner rules to NOT apply when the user was
listed as an owner of one of the applicable rules, essentially assuming
their approval as implied due to them taking the git action the check is
being applied to. However, this is incorrect, as we still want to push
users towards the MR workflow, even when they are in fact an owner who
_could_ approve the change.

ee/changelogs/unreleased/kerrizor-remove-code-owner-user-identity-check.yml

+---
+title: Remove check for user being an applicable code owner
+merge_request: 31768
+author:
+type: fixed

ee/lib/ee/gitlab/checks/diff_check.rb

@@ -39,7 +39,6 @@ module EE
             loader = ::Gitlab::CodeOwners::Loader.new(project, branch_name, paths)
 
             return if loader.entries.blank?
-            return if loader.members.include?(change_access.user_access.user)
 
             assemble_error_msg_for_codeowner_matches(loader)
           end

ee/spec/lib/gitlab/checks/diff_check_spec.rb

@@ -8,6 +8,12 @@ describe Gitlab::Checks::DiffCheck do
   include_context 'push rules checks context'
 
   describe '#validate!' do
+    shared_examples_for "returns codeowners validation message" do
+      it "returns an error message" do
+        expect(validation_result).to include("Pushes to protected branches")
+      end
+    end
+
     context 'no push rules active' do
       let_it_be(:push_rule) { create(:push_rule) }
 
@@ -55,11 +61,6 @@ describe Gitlab::Checks::DiffCheck do
           )
         end
 
-        context 'and the user is not listed as a codeowner' do
-          before do
-            stub_feature_flags(sectional_codeowners: false)
-          end
-
         it "returns an error message" do
           expect { diff_check.validate! }.to raise_error do |error|
             expect(error).to be_a(Gitlab::GitAccess::ForbiddenError)
@@ -68,121 +69,13 @@ describe Gitlab::Checks::DiffCheck do
         end
       end
 
-        context 'and the user is listed as a codeowner' do
-          # `user` is set as the owner of the incoming change by the shared
-          #   context found in 'push rules checks context'
-          let(:codeowner_content) { "* @#{user.username}" }
-
-          it "does not return an error message" do
-            expect { diff_check.validate! }.not_to raise_error
-          end
-        end
-      end
-
       context "the MR contains a matching file path" do
         let(:validation_result) do
           subject.send(:validate_code_owners).call(["docs/CODEOWNERS", "README"])
         end
 
-        context "and the user is not listed as a code owner" do
-          before do
-            stub_feature_flags(sectional_codeowners: false)
-          end
-
-          context "for a non-web-based request" do
-            it "returns an error message" do
-              expect(validation_result).to include("Pushes to protected branches")
-              expect(validation_result).to include("\n")
-            end
-          end
-
-          context "for a web-based request" do
-            before do
-              allow(subject).to receive(:updated_from_web?).and_return(true)
-            end
-
-            it "returns an error message with newline chars removed" do
-              expect(validation_result).to include("Pushes to protected branches")
-              expect(validation_result).not_to include("\n")
-            end
-          end
-        end
-
-        context "and the user is listed as a code owner" do
-          # `user` is set as the owner of the incoming change by the shared
-          #   context found in 'push rules checks context'
-          let(:codeowner_content) { "* @#{user.username}" }
-
-          it "returns nil" do
-            expect(validation_result).to be_nil
-          end
-        end
-
-        context "when the codeowner entity is a group" do
-          let(:group) { create(:group) }
-          let(:project) { create(:project, :repository, namespace: group) }
-          let(:codeowner_content) do
-            <<~CODEOWNERS
-            *.rb @#{code_owner.username}
-            docs/CODEOWNERS @#{group.full_path}
-            *.js.coffee @#{group.full_path}
-            CODEOWNERS
-          end
-
-          before do
-            stub_feature_flags(sectional_codeowners: false)
-          end
-
-          shared_examples_for "returns nil" do
-            it "returns nil" do
-              expect(validation_result).to be_nil
-            end
-          end
-
-          shared_examples_for "returns codeowners validation message" do
-            it "returns an error message" do
-              expect(validation_result).to include("Pushes to protected branches")
-            end
-          end
-
-          context "and the project is under a subgroup" do
-            let(:subgroup) { create(:group, parent: group) }
-            let(:project) { create(:project, :repository, namespace: subgroup) }
-
-            context "and the user is part of the codeowning-group" do
-              before do
-                group.add_developer(user)
-              end
-
-              it_behaves_like "returns nil"
-            end
-
-            context "and the user is not part of the codeowning-group" do
-              it_behaves_like "returns codeowners validation message"
-            end
-
-            context "with :codeowners_match_ancestor_groups disabled" do
-              before do
-                stub_feature_flags(codeowners_match_ancestor_groups: false)
-              end
-
-              it_behaves_like "returns codeowners validation message"
-            end
-          end
-
-          context "and the user is part of the codeowning-group" do
-            before do
-              group.add_developer(user)
-            end
-
-            it_behaves_like "returns nil"
-          end
-
-          context "and the user is not part of the codeowning-group" do
         it_behaves_like "returns codeowners validation message"
       end
-        end
-      end
 
       context "the MR doesn't contain a matching file path" do
         it "returns nil" do