ensure deleted object is entry

Some code is coming that puts a new D-Bus object on the
/xyz/openbmc_project/logging/ path. The addCallback() interface
correctly verified the path was an entry but the delCallBack() path did
not.

Tested:
Verified with this change that phosphor-dump-manager no longer core
dumps when one of these new objects is deleted.

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: Ie702e52b97bc484a44543719e10eab573f8ecb73