Remove obsoleted sectional_codeowners FF in tests

As part of work to iteratively build section codeowners I did
development work behind a feature flag sectional_codeowners which can
be removed now that sectional codeowner processing has aligned with the
legacy processing - feeding a sectional or legacy formatted file through
either the legacy or sectional processing routines will return the same
data (the only difference being that sectional data through the
sectional parser will add to the Entry object the section in which it
was found.)

ee/spec/features/projects/merge_requests/user_edits_merge_request_spec.rb

@@ -32,8 +32,6 @@ describe 'Projects > Merge Requests > User edits a merge request' do
     let(:ruby_owner) { create(:user, username: 'ruby-owner') }
 
     before do
-      stub_feature_flags(sectional_codeowners: false)
-
       project.add_developer(ruby_owner)
       project.repository.create_file(user, 'ruby.rb', '# a ruby file',
                                      message: 'Add a ruby file',

ee/spec/features/projects/view_blob_with_code_owners_spec.rb

@@ -22,7 +22,6 @@ describe 'File blob > Code owners', :js do
     context 'when the feature is available' do
       before do
         stub_licensed_features(code_owners: true)
-        stub_feature_flags(sectional_codeowners: false)
       end
 
       it 'shows the code owners related to a file' do

ee/spec/lib/gitlab/code_owners_spec.rb

@@ -26,7 +26,6 @@ describe Gitlab::CodeOwners do
     context 'when the feature is available' do
       before do
         stub_licensed_features(code_owners: true)
-        stub_feature_flags(sectional_codeowners: false)
       end
 
       it 'returns users for a blob' do
@@ -60,7 +59,6 @@ describe Gitlab::CodeOwners do
 
     before do
       stub_licensed_features(code_owners: true)
-      stub_feature_flags(sectional_codeowners: false)
     end
 
     it "return equivalent results" do
@@ -91,7 +89,6 @@ describe Gitlab::CodeOwners do
     context 'when the feature is available' do
       before do
         stub_licensed_features(code_owners: true)
-        stub_feature_flags(sectional_codeowners: false)
       end
 
       it 'returns owners for merge request' do