Remove preset_root_ancestor_for_labels flag

This flag was tested in production w/o issues.

app/finders/labels_finder.rb

@@ -102,7 +102,7 @@ class LabelsFinder < UnionFinder
 
       # Because we are sure that all groups are in the same hierarchy tree
       # we can preset root group for all of them to optimize permission checks
-      Group.preset_root_ancestor_for(groups) if Feature.enabled?(:preset_root_ancestor_for_labels, group)
+      Group.preset_root_ancestor_for(groups)
 
       groups_user_can_read_labels(groups).map(&:id)
     end

changelogs/unreleased/preset_ancestor.yml

+---
+title: Optimized loading of descendant group labels
+merge_request: 53356
+author:
+type: performance

config/feature_flags/development/preset_root_ancestor_for_labels.yml

----
-name: preset_root_ancestor_for_labels
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52236
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/299521
-milestone: '13.9'
-type: development
-group: group::plan
-default_enabled: false

ee/spec/requests/groups/labels_spec.rb

@@ -21,20 +21,6 @@ RSpec.describe 'view audit events' do
       expect { send_request }.not_to exceed_all_query_limit(control)
     end
 
-    context 'when preset_root_ancestor_for_labels flag is disabled' do
-      before do
-        stub_feature_flags(preset_root_ancestor_for_labels: false)
-      end
-
-      it 'does additional N+1 DB queries', :request_store do
-        control = ActiveRecord::QueryRecorder.new(skip_cached: false) { send_request }
-
-        create_list(:group, 3, parent: group)
-
-        expect { send_request }.to exceed_all_query_limit(control)
-      end
-    end
-
     def send_request
       get group_labels_path(group, include_descendant_groups: true, format: :json)
     end