Add forgotten IgnorableColumn in Geo::RepositoryDeletedEvent

In gitlab-org/gitlab-ee!5389 the use of repository_storage_path was
removed. We ignored those columns in the geo event log tables, but for
the `geo_repository_deleted_events` table, this was forgotten.

gitlab-org/gitlab-ee!5468 is prepared to actually remove the columns
with a migration.

ee/app/models/geo/repository_deleted_event.rb

 module Geo
   class RepositoryDeletedEvent < ActiveRecord::Base
     include Geo::Model
+    include IgnorableColumn
+
+    ignore_column :repository_storage_path
 
     belongs_to :project