Destroy synced linked work items even when the FF is disabled

When removing a related epic that has a synced work item, also
destroy the RelatedWorkItemLinks record

ee/app/services/epics/related_epic_links/destroy_service.rb

@@ -71,9 +71,7 @@ def epic_is_link_target?
       end
 
       def sync_to_work_item!
-        return unless epic.group.epic_synced_with_work_item_enabled?
         return unless epic.work_item
-
         return unless source.issue_id && target.issue_id
 
         item_ids = epic_is_link_source? ? [target.issue_id] : [source.issue_id]

ee/spec/services/epics/related_epic_links/destroy_service_spec.rb

@@ -37,9 +37,9 @@
           stub_feature_flags(epic_creation_with_synced_work_item: false)
         end
 
-        it 'removes the epic but not the work item relation' do
+        it 'removes the epic and the work item relation' do
           expect { subject }.to change { issuable_link.class.count }.by(-1)
-          .and not_change { WorkItems::RelatedWorkItemLink.count }
+            .and change { WorkItems::RelatedWorkItemLink.count }.by(-1)
         end
       end