Fix to set None as old AT content type UID before object is deleted

@pbauer Do you remember having any AT relations issues with the current p.a.contenttypes migration? This patch of removing the UID from the old object before it's been deleted and all deletion related events are triggered was required for me to avoid resulting broken relations_catalog after the migration (it was full of relations pointing to MIGRATION-objects).

I'm haven't tested this outside my own migrations yet.