Don't traceback when unversioning a directory.

* bzrlib/tests/test_workingtree_4.py:
(TestInventoryCoherency.test_inventory_is_synced_when_unversioning_a_dir):
Reproduce bug #494221 where we try to unversion a children even if
the remove_recursive_id for the parent has already done it.

* bzrlib/workingtree_4.py:
(DirStateWorkingTree.unversion): Don't try to unversion an entry
that is not present.