Sort the dependent objects before deletion in DROP OWNED BY.

Sort the dependent objects before deletion in DROP OWNED BY.

This finishes a task we left undone in commit f1ad067fc, by extending
the delete-in-descending-OID-order rule to deletions triggered by
DROP OWNED BY.  We've coped with machine-dependent deletion orders
one time too many, and the new issues caused by Peter G's recent
nbtree hacking seem like the last straw.

Discussion: https://postgr.es/m/E1h6eep-0001Mw-Vd@gemulon.postgresql.org