Enable merge requests discussions cache

This removes the merge_request_discussions_cache feature flag.

The feature flag has been enabled on production for a couple of
months now and performance improvements were seen and bugs were
fixed.

Changelog: changed

app/controllers/concerns/issuable_actions.rb

@@ -158,7 +158,7 @@ module IssuableActions
 
     discussions = Discussion.build_collection(notes, issuable)
 
-    if issuable.is_a?(MergeRequest) && Feature.enabled?(:merge_request_discussion_cache, issuable.target_project, default_enabled: :yaml)
+    if issuable.is_a?(MergeRequest)
       cache_context = [current_user&.cache_key, project.team.human_max_access(current_user&.id)].join(':')
 
       render_cached(discussions, with: discussion_serializer, cache_context: -> (_) { cache_context }, context: self)

config/feature_flags/development/merge_request_discussion_cache.yml

----
-name: merge_request_discussion_cache
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64688
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/335799
-milestone: '14.1'
-type: development
-group: group::code review
-default_enabled: false

spec/requests/projects/merge_requests_discussions_spec.rb

@@ -251,16 +251,6 @@ RSpec.describe 'merge requests discussions' do
           let(:changed_notes) { [first_note, second_note] }
         end
       end
-
-      context 'when merge_request_discussion_cache is disabled' do
-        before do
-          stub_feature_flags(merge_request_discussion_cache: false)
-        end
-
-        it_behaves_like 'cache miss' do
-          let(:changed_notes) { [first_note, second_note] }
-        end
-      end
     end
   end
 end