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_objmrtree_destroy_u(COMPS_Object *obj) {
    comps_objmrtree_destroy((COMPS_ObjMRTree*)obj);
}

void comps_objmrtree_destroy_u(COMPS_Object *obj) {
    comps_objmrtree_destroy((COMPS_ObjMRTree*)obj);
}
