Add forgotten IgnorableColumn in Geo::RepositoryDeletedEvent
What does this MR do?
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.