Fix field not being preloaded in multisite setup.

plugin.rb

@@ -33,12 +33,7 @@ after_initialize do
     end
   end
 
-  # We can remove this check once this method is stable
-  if respond_to?(:add_preloaded_topic_list_custom_field)
-    add_preloaded_topic_list_custom_field('assigned_to_id')
-  else
-    TopicList.preloaded_custom_fields << "assigned_to_id"
-  end
+  TopicList.preloaded_custom_fields << "assigned_to_id"
 
   TopicList.on_preload do |topics, topic_list|
     if SiteSetting.assign_enabled?