Repository#exists? can only be queried with Gitaly

Has been in opt out for 5 months, and within GitLab been in production
for longer than that.

No code needs to be migrated as this is implemented in GoLang over at
Gitaly.

Closes https://gitlab.com/gitlab-org/gitaly/issues/314

changelogs/unreleased/zj-repository-exist-mandatory.yml

+---
+title: Repository#exists? is always executed through Gitaly
+merge_request:
+author:
+type: performance

lib/gitlab/git/repository.rb

@@ -142,15 +142,7 @@ module Gitlab
       end
 
       def exists?
-        Gitlab::GitalyClient.migrate(:repository_exists, status: Gitlab::GitalyClient::MigrationStatus::OPT_OUT) do |enabled|
-          if enabled
         gitaly_repository_client.exists?
-          else
-            circuit_breaker.perform do
-              File.exist?(File.join(path, 'refs'))
-            end
-          end
-        end
       end
 
       # Returns an Array of branch names

spec/models/repository_spec.rb

@@ -1224,15 +1224,15 @@ describe Repository do
     end
   end
 
-  shared_examples 'repo exists check' do
+  describe '#exists?' do
     it 'returns true when a repository exists' do
-      expect(repository.exists?).to eq(true)
+      expect(repository.exists?).to be(true)
     end
 
     it 'returns false if no full path can be constructed' do
       allow(repository).to receive(:full_path).and_return(nil)
 
-      expect(repository.exists?).to eq(false)
+      expect(repository.exists?).to be(false)
     end
 
     context 'with broken storage', :broken_storage do
@@ -1242,16 +1242,6 @@ describe Repository do
     end
   end
 
-  describe '#exists?' do
-    context 'when repository_exists is disabled' do
-      it_behaves_like 'repo exists check'
-    end
-
-    context 'when repository_exists is enabled', :skip_gitaly_mock do
-      it_behaves_like 'repo exists check'
-    end
-  end
-
   describe '#has_visible_content?' do
     before do
       # If raw_repository.has_visible_content? gets called more than once then