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.
COMPS_HSList* comps_objmrtree_keys(COMPS_ObjMRTree * rt) {
    return __comps_objmrtree_all(rt, 0);
}

COMPS_HSList* comps_objmrtree_keys(COMPS_ObjMRTree * rt) {
    return __comps_objmrtree_all(rt, 0);
}
