Remove obsolete resource method in controllers

This method was used by controllers using the `Translatable` concern. We
forgot to remove it in commit 71601bd3f.

app/controllers/admin/admin_notifications_controller.rb

@@ -69,8 +69,4 @@ class Admin::AdminNotificationsController < Admin::BaseController
     def allowed_params
       [:link, :segment_recipient, translation_params(AdminNotification)]
     end
-
-    def resource
-      AdminNotification.find(params[:id])
-    end
 end