bus: do not unref the added child bus on realize

When the parent bus removes the child property, it takes care of
removing the added reference, in object_finalize_child_property().

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20180531195119.22021-2-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>