Use .eradiate for tombstones

Provides a way to re-use URIs by removing the `fcr:tombstone` resources that Fedora leaves behind to prevent their re-use. Technically, this is way to circumvent LDP rules. Also note that in typical operations, re-using an existing URI will result in a Ldp::Gone error. If everyone's comfortable with that error, no more needs to be done.
I'd prefer we pass ghost as a parameter to delete_tombstone instead of an ivar