Fix: release the reference held by a child to its former parent

The bt_object class' bt_object_set_parent() increments the reference
count of the parent since the child now holds a reference to it.
This assumes that a child is externally visible when this call is
made (as is always the case currently).

The parent of a child can be changed in the future. In this case,
we want to ensure the reference it held to its former parent is
released.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>