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/site_customization/pages_controller.rb

@@ -43,8 +43,4 @@ class Admin::SiteCustomization::PagesController < Admin::SiteCustomization::Base
 
       [*attributes, translation_params(SiteCustomization::Page)]
     end
-
-    def resource
-      SiteCustomization::Page.find(params[:id])
-    end
 end