Doc: remove stale entry for removed aclitem[] ~ aclitem operator.

Doc: remove stale entry for removed aclitem[] ~ aclitem operator.

Commit 2f70fdb06 removed the deprecated containment operator
~(aclitem[],aclitem) from the catalogs, but missed removing its entry
from the documentation.  (Arguably the blame should fall on c62dd80cd,
which added this entry in contravention of the longstanding policy
that we don't document deprecated aliases in the first place.)

Author: Shinya Kato <shinya11.kato@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAOzEurQSyR5psWukyhUz1LtxyO55C2Vfp0Fmt8w2jGKxhszQmQ@mail.gmail.com
Backpatch-through: 14