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/banners_controller.rb

@@ -44,8 +44,4 @@ class Admin::BannersController < Admin::BaseController
        translation_params(Banner),
        web_section_ids: []]
     end
-
-    def resource
-      @banner ||= Banner.find(params[:id])
-    end
 end