Remove merge_request_cached_pipeline_serializer FF

The FF has been enabled by default for a while
Let's remove it

app/serializers/merge_request_poll_cached_widget_entity.rb

@@ -46,9 +46,7 @@ class MergeRequestPollCachedWidgetEntity < IssuableEntity
     end
   end
 
-  expose :actual_head_pipeline, as: :pipeline, if: -> (mr, _) {
-    Feature.enabled?(:merge_request_cached_pipeline_serializer, mr.project) && presenter(mr).can_read_pipeline?
-  } do |merge_request, options|
+  expose :actual_head_pipeline, as: :pipeline, if: -> (mr, _) { presenter(mr).can_read_pipeline? } do |merge_request, options|
     MergeRequests::PipelineEntity.represent(merge_request.actual_head_pipeline, options)
   end
 

app/serializers/merge_request_poll_widget_entity.rb

@@ -19,12 +19,6 @@ class MergeRequestPollWidgetEntity < Grape::Entity
   # User entities
   expose :merge_user, using: UserEntity
 
-  expose :actual_head_pipeline, as: :pipeline, if: -> (mr, _) {
-    Feature.disabled?(:merge_request_cached_pipeline_serializer, mr.project) && presenter(mr).can_read_pipeline?
-  } do |merge_request, options|
-    MergeRequests::PipelineEntity.represent(merge_request.actual_head_pipeline, options)
-  end
-
   expose :merge_pipeline, if: ->(mr, _) { mr.merged? && can?(request.current_user, :read_pipeline, mr.target_project)} do |merge_request, options|
     MergeRequests::PipelineEntity.represent(merge_request.merge_pipeline, options)
   end

changelogs/unreleased/id-remove-cached-pipeline-serializer-ff.yml

+---
+title: Move pipelines calculation from widget.json to cached_widget.json
+merge_request: 57822
+author:
+type: performance

config/feature_flags/development/merge_request_cached_pipeline_serializer.yml

----
-name: merge_request_cached_pipeline_serializer
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38273
-rollout_issue_url: 
-milestone: '13.5'
-type: development
-group: group::code review
-default_enabled: false

spec/serializers/merge_request_poll_cached_widget_entity_spec.rb

@@ -248,14 +248,6 @@ RSpec.describe MergeRequestPollCachedWidgetEntity do
 
           expect(subject[:pipeline]).to eq(pipeline_payload)
         end
-
-        context 'when merge_request_cached_pipeline_serializer is disabled' do
-          it 'does not return pipeline' do
-            stub_feature_flags(merge_request_cached_pipeline_serializer: false)
-
-            expect(subject[:pipeline]).to be_nil
-          end
-        end
       end
 
       context 'when user does not have access to pipelines' do

spec/serializers/merge_request_poll_widget_entity_spec.rb

@@ -226,19 +226,6 @@ RSpec.describe MergeRequestPollWidgetEntity do
           expect(subject[:pipeline]).to be_nil
         end
 
-        context 'when merge_request_cached_pipeline_serializer is disabled' do
-          it 'returns detailed info about pipeline' do
-            stub_feature_flags(merge_request_cached_pipeline_serializer: false)
-
-            pipeline_payload =
-              MergeRequests::PipelineEntity
-                .represent(pipeline, request: req)
-                .as_json
-
-            expect(subject[:pipeline]).to eq(pipeline_payload)
-          end
-        end
-
         it 'returns ci_status' do
           expect(subject[:ci_status]).to eq('pending')
         end