Remove early_prepare_for_mergeability feature flag

The feature flag was enabled on production for more than a week
now. Looking at https://log.gprd.gitlab.net/goto/11419880-87d8-11ec-a649-b7cbb8e4f62e,
the errors that are occuring are still related to Gitaly timeout.

Those errors happens after the MR is prepared for mergeability
which is why we prepare for mergeability earlier. This way even if
the rest of the job fails, the MR will already be checked for
mergeability.

This removes the feature flag and enables the early check for all.

Changelog: changed

app/services/merge_requests/after_create_service.rb

@@ -5,9 +5,8 @@ module MergeRequests
     include Gitlab::Utils::StrongMemoize
 
     def execute(merge_request)
-      prepare_for_mergeability(merge_request) if early_prepare_for_mergeability?(merge_request)
+      prepare_for_mergeability(merge_request)
       prepare_merge_request(merge_request)
-      check_mergeability(merge_request) unless early_prepare_for_mergeability?(merge_request)
     end
 
     private
@@ -26,11 +25,6 @@ module MergeRequests
 
       notification_service.new_merge_request(merge_request, current_user)
 
-      unless early_prepare_for_mergeability?(merge_request)
-        create_pipeline_for(merge_request, current_user)
-        merge_request.update_head_pipeline
-      end
-
       merge_request.diffs(include_stats: false).write_cache
       merge_request.create_cross_references!(current_user)
 
@@ -49,12 +43,6 @@ module MergeRequests
       LinkLfsObjectsService.new(project: merge_request.target_project).execute(merge_request)
     end
 
-    def early_prepare_for_mergeability?(merge_request)
-      strong_memoize("early_prepare_for_mergeability_#{merge_request.target_project_id}".to_sym) do
-        Feature.enabled?(:early_prepare_for_mergeability, merge_request.target_project)
-      end
-    end
-
     def check_mergeability(merge_request)
       return unless merge_request.preparing?
 

config/feature_flags/development/early_prepare_for_mergeability.yml

----
-name: early_prepare_for_mergeability
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75402
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/346667
-milestone: '14.6'
-type: development
-group: group::code review
-default_enabled: false

spec/services/merge_requests/after_create_service_spec.rb

@@ -108,17 +108,6 @@ RSpec.describe MergeRequests::AfterCreateService do
           expect { execute_service }.to raise_error(StandardError)
           expect(merge_request.reload).to be_preparing
         end
-
-        context 'when early_prepare_for_mergeability feature flag is disabled' do
-          before do
-            stub_feature_flags(early_prepare_for_mergeability: false)
-          end
-
-          it 'does not mark the merge request as unchecked' do
-            expect { execute_service }.to raise_error(StandardError)
-            expect(merge_request.reload).to be_preparing
-          end
-        end
       end
 
       context 'when preparing merge request fails' do
@@ -134,17 +123,6 @@ RSpec.describe MergeRequests::AfterCreateService do
           expect(merge_request).to receive(:check_mergeability).with(async: true)
           expect { execute_service }.to raise_error(StandardError)
         end
-
-        context 'when early_prepare_for_mergeability feature flag is disabled' do
-          before do
-            stub_feature_flags(early_prepare_for_mergeability: false)
-          end
-
-          it 'does not mark the merge request as unchecked' do
-            expect { execute_service }.to raise_error(StandardError)
-            expect(merge_request.reload).to be_preparing
-          end
-        end
       end
     end