Remove merge_ref_auto_sync flag

This flag was originally added in 12.0

app/services/merge_requests/mergeability_check_service.rb

@@ -64,10 +64,6 @@ module MergeRequests
         return ServiceResponse.error(message: 'Merge request is not mergeable')
       end
 
-      unless merge_ref_auto_sync_enabled?
-        return ServiceResponse.error(message: 'Merge ref is outdated due to disabled feature')
-      end
-
       unless payload.fetch(:merge_ref_head)
         return ServiceResponse.error(message: 'Merge ref cannot be updated')
       end
@@ -142,7 +138,6 @@ module MergeRequests
     #
     # Returns true if the merge-ref does not exists or is out of sync.
     def outdated_merge_ref?
-      return false unless merge_ref_auto_sync_enabled?
       return false unless merge_request.open?
 
       return true unless ref_head = merge_request.merge_ref_head
@@ -157,17 +152,11 @@ module MergeRequests
     end
 
     def merge_to_ref
-      return true unless merge_ref_auto_sync_enabled?
-
       params = { allow_conflicts: Feature.enabled?(:display_merge_conflicts_in_diff, project) }
       result = MergeRequests::MergeToRefService.new(project, merge_request.author, params).execute(merge_request)
       result[:status] == :success
     end
 
-    def merge_ref_auto_sync_enabled?
-      Feature.enabled?(:merge_ref_auto_sync, project, default_enabled: true)
-    end
-
     def merge_ref_auto_sync_lock_enabled?
       Feature.enabled?(:merge_ref_auto_sync_lock, project, default_enabled: true)
     end

config/feature_flags/development/merge_ref_auto_sync.yml

----
-name: merge_ref_auto_sync
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/29569
-rollout_issue_url: 
-milestone: '12.1'
-type: development
-group: group::code review
-default_enabled: true

spec/services/merge_requests/mergeability_check_service_spec.rb

@@ -167,25 +167,6 @@ RSpec.describe MergeRequests::MergeabilityCheckService, :clean_gitlab_redis_shar
       end
     end
 
-    context 'disabled merge ref sync feature flag' do
-      before do
-        stub_feature_flags(merge_ref_auto_sync: false)
-      end
-
-      it 'returns error and no payload' do
-        result = subject
-
-        expect(result).to be_a(ServiceResponse)
-        expect(result.error?).to be(true)
-        expect(result.message).to eq('Merge ref is outdated due to disabled feature')
-        expect(result.payload).to be_empty
-      end
-
-      it 'ignores merge-ref and updates merge status' do
-        expect { subject }.to change(merge_request, :merge_status).from('unchecked').to('can_be_merged')
-      end
-    end
-
     context 'when broken' do
       before do
         expect(merge_request).to receive(:broken?) { true }
@@ -305,28 +286,6 @@ RSpec.describe MergeRequests::MergeabilityCheckService, :clean_gitlab_redis_shar
     context 'recheck enforced' do
       subject { described_class.new(merge_request).execute(recheck: true) }
 
-      context 'when MR is mergeable and merge-ref auto-sync is disabled' do
-        before do
-          stub_feature_flags(merge_ref_auto_sync: false)
-          merge_request.mark_as_mergeable!
-        end
-
-        it 'returns ServiceResponse.error' do
-          result = subject
-
-          expect(result).to be_a(ServiceResponse)
-          expect(result.error?).to be(true)
-          expect(result.message).to eq('Merge ref is outdated due to disabled feature')
-          expect(result.payload).to be_empty
-        end
-
-        it 'merge status is not changed' do
-          subject
-
-          expect(merge_request.merge_status).to eq('can_be_merged')
-        end
-      end
-
       context 'when MR is marked as mergeable, but repo is not mergeable and MR is not opened' do
         before do
           # Making sure that we don't touch the merge-status after