qdev: Drop misleading qbus_free() function

Same reasoning as commit 02a5c4c97422b40034f31265e0f139f7846172a8
("qdev: Drop misleading qdev_free() function").  The qbus_free()
function removes the child from the namespace and decrements the
reference count.  It does not, however, guarantee to free the child
since the refcount may still be held.

Just call object_unparent() directly.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>