Unset nodeIdentifier on Delete

I am not completely sure this is an actual bug and if so, if this is the correct fix. But it works for me ;-)

The bug I ran into can be reproduced like this:

1. Use an existing workspace
2. Delete a node from it (that has a jcr:uuid)
3. `$session->save()`
4. Recreate the node with a new reference
5. `$session->save()`
6. Use that node as reference for another (new) node

You will now get an `PHPCR\ReferentialIntegrityException`, stating that the reference you gave the does not exist.
That failed because Client.php holds an internal list of UUID's and found the UUID for the deleted Node there.