Fix UAF in comps_objmrtree_unite function

The added field is not used at all in many places and it is probably the
left-over of some copy-paste.
void comps_objrtree_create_u(COMPS_Object * obj, COMPS_Object **args) {
    (void)args;
    comps_objrtree_create((COMPS_ObjRTree*)obj, NULL);
}

void comps_objrtree_create_u(COMPS_Object * obj, COMPS_Object **args) {
    (void)args;
    comps_objrtree_create((COMPS_ObjRTree*)obj, NULL);
}
