Optimize Group#for_epics scope

Remove optimized_groups_user_can_read_epics_method feature flag and keep
the optimized code in the codebase.

Closes https://gitlab.com/gitlab-org/gitlab/issues/37748

ee/app/models/ee/group.rb

@@ -75,12 +75,8 @@ module EE
       end
 
       scope :for_epics, ->(epics) do
-        if ::Feature.enabled?(:optimized_groups_user_can_read_epics_method)
-          epics_query = epics.select(:group_id)
-          joins("INNER JOIN (#{epics_query.to_sql}) as epics on epics.group_id = namespaces.id")
-        else
-          where(id: epics.select(:group_id))
-        end
+        epics_query = epics.select(:group_id)
+        joins("INNER JOIN (#{epics_query.to_sql}) as epics on epics.group_id = namespaces.id")
       end
 
       state_machine :ldap_sync_status, namespace: :ldap_sync, initial: :ready do

ee/changelogs/unreleased/37748-clean-up-optimized_groups_user_can_read_epics_method-ff.yml

+---
+title: Optimize Group#for_epics scope
+merge_request: 22375
+author:
+type: performance

ee/spec/models/group_spec.rb

@@ -77,29 +77,9 @@ describe Group do
       let_it_be(:epic1) { create(:epic) }
       let_it_be(:epic2) { create(:epic) }
 
-      shared_examples '.for_epics examples' do
-        it 'returns groups only for selected epics' do
-          epics = ::Epic.where(id: epic1)
-          expect(described_class.for_epics(epics)).to contain_exactly(epic1.group)
-        end
-      end
-
-      context 'with `optimized_groups_user_can_read_epics_method` feature flag' do
-        before do
-          stub_feature_flags(optimized_groups_user_can_read_epics_method: flag_state)
-        end
-
-        context 'enabled' do
-          let(:flag_state) { true }
-
-          include_examples '.for_epics examples'
-        end
-
-        context 'disabled' do
-          let(:flag_state) { false }
-
-          include_examples '.for_epics examples'
-        end
+      it 'returns groups only for selected epics' do
+        epics = ::Epic.where(id: epic1)
+        expect(described_class.for_epics(epics)).to contain_exactly(epic1.group)
       end
     end
   end