Remove obsolete RECHECK keyword completely

Remove obsolete RECHECK keyword completely

This used to be part of CREATE OPERATOR CLASS and ALTER OPERATOR
FAMILY, but it has done nothing (except issue a NOTICE) since
PostgreSQL 8.4.  Commit 30e7c175b81 removed support for dumping from
pre-9.2 servers, so this no longer serves any need.

This now removes it completely, and you'd get a normal parse error if
you used it.

Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://www.postgresql.org/message-id/flat/113ef2d2-3657-4353-be97-f28fceddbca1%40eisentraut.org