Fix issue with `prevent_deletion` subscriber not being able to acquire membership_tool

Fixes issue with `prevent_deletion` subscriber not being able to acquire `membership_tool` because of broken acquisition during Plone site deletion.

During Plone site deletion, on some objects of type `opengever.document.document`, `obj.portal_membership` fails to acquire the membership tool, even though it still available on the Plone site. `event.object` on the other hand seems to give us a reliable reference to the Plone site and a valid acquisition conext.