Remove cached_mr_widget feature flag

As discussed in https://gitlab.com/gitlab-org/gitlab/-/issues/332966#note_683932353,
we are no longer pursuing the approach. Because of that, we're
removing the feature flag.

Changelog: removed

app/views/projects/merge_requests/_widget.html.haml

-= cache_if(Feature.enabled?(:cached_mr_widget, @merge_request.project), [@merge_request.project, @merge_request, current_user], expires_in: 10.minutes) do
-  - artifacts_endpoint_placeholder = ':pipeline_artifacts_id'
+- artifacts_endpoint_placeholder = ':pipeline_artifacts_id'
 
-  = javascript_tag do
+= javascript_tag do
   :plain
     window.gl = window.gl || {};
     window.gl.mrWidgetData = #{serialize_issuable(@merge_request, serializer: 'widget', issues_links: true)}
@@ -23,4 +22,4 @@
     window.gl.mrWidgetData.user_preferences_gitpod_path = '#{profile_preferences_path(anchor: 'user_gitpod_enabled')}';
     window.gl.mrWidgetData.user_profile_enable_gitpod_path = '#{profile_path(user: { gitpod_enabled: true })}';
 
-  #js-vue-mr-widget.mr-widget
+#js-vue-mr-widget.mr-widget

config/feature_flags/development/cached_mr_widget.yml

----
-name: cached_mr_widget
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61584
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/330803
-milestone: '13.12'
-type: development
-group: group::code review
-default_enabled: false