Introduce an optimized labels query on group transfer

This MR enables the optimized query for fetching issue and
merge request labels to be transfered when a group or a
project is transfered.

Changelog: performance

app/services/labels/transfer_service.rb

@@ -50,32 +50,17 @@ module Labels
 
     # rubocop: disable CodeReuse/ActiveRecord
     def group_labels_applied_to_issues
-      @labels_applied_to_issues ||= if use_optimized_group_labels_query?
-                                      Label.joins(:issues)
+      @labels_applied_to_issues ||= Label.joins(:issues)
         .joins("INNER JOIN namespaces on namespaces.id = labels.group_id AND namespaces.type = 'Group'" )
         .where(issues: { project_id: project.id }).reorder(nil)
-                                    else
-                                      Label.joins(:issues).where(
-                                        issues: { project_id: project.id },
-                                        labels: { group_id: old_group.self_and_ancestors }
-                                      )
-                                    end
     end
     # rubocop: enable CodeReuse/ActiveRecord
 
     # rubocop: disable CodeReuse/ActiveRecord
     def group_labels_applied_to_merge_requests
-      @labels_applied_to_mrs ||= if use_optimized_group_labels_query?
-                                   Label.joins(:merge_requests)
+      @labels_applied_to_mrs ||= Label.joins(:merge_requests)
         .joins("INNER JOIN namespaces on namespaces.id = labels.group_id AND namespaces.type = 'Group'" )
         .where(merge_requests: { target_project_id: project.id }).reorder(nil)
-                                 else
-                                   Label.joins(:merge_requests)
-                                     .where(
-                                       merge_requests: { target_project_id: project.id },
-                                       labels: { group_id: old_group.self_and_ancestors }
-                                     )
-                                 end
     end
     # rubocop: enable CodeReuse/ActiveRecord
 
@@ -99,9 +84,5 @@ module Labels
         .update_all(label_id: new_label_id)
     end
     # rubocop: enable CodeReuse/ActiveRecord
-
-    def use_optimized_group_labels_query?
-      Feature.enabled?(:use_optimized_group_labels_query, project.root_namespace, default_enabled: :yaml)
-    end
   end
 end

config/feature_flags/development/use_optimized_group_labels_query.yml

----
-name: use_optimized_group_labels_query
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/73501
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/344957
-milestone: '14.5'
-type: development
-group: group::workspace
-default_enabled: true

spec/services/labels/transfer_service_spec.rb

@@ -109,15 +109,5 @@ RSpec.describe Labels::TransferService do
     end
   end
 
-  context 'with use_optimized_group_labels_query FF on' do
   it_behaves_like 'transfer labels'
-  end
-
-  context 'with use_optimized_group_labels_query FF off' do
-    before do
-      stub_feature_flags(use_optimized_group_labels_query: false)
-    end
-
-    it_behaves_like 'transfer labels'
-  end
 end