libubus: Fix deletion from context's object AVL tree when removing object

libubus: Fix deletion from context's object AVL tree when removing object

Objects are stored in the ubus context in an AVL tree.  An AVL tree
node contains a pointer to a key value.  For the ubus context, this
points to the id member of the object structure.  In
ubus_remove_object_cb, the id member is set to zero and then after,
avl_delete is called and fails.  To fix this, we call avl_delete
before setting the object id to zero.

Signed-off-by: Bob Ham <bob.ham@tomltd.co.uk>