Remove the :mr_list_api_skip_merge_status_recheck feature flag

This was introduced "just in case", but we've had no complaints so now
is a good time to remove it.

lib/api/merge_requests.rb

@@ -92,10 +92,8 @@ module API
           options[:with] = Entities::MergeRequestSimple
         else
           options[:issuable_metadata] = Gitlab::IssuableMetadata.new(current_user, merge_requests).data
-          if Feature.enabled?(:mr_list_api_skip_merge_status_recheck, default_enabled: true)
           options[:skip_merge_status_recheck] = !declared_params[:with_merge_status_recheck]
         end
-        end
 
         options
       end