Remove old RULE privilege completely.

Remove old RULE privilege completely.

The RULE privilege for tables was removed in v8.2, but for backward
compatibility, GRANT/REVOKE and privilege functions like
has_table_privilege continued to accept the RULE keyword without
any effect.

After discussions on pgsql-hackers, it was agreed that this compatibility
is no longer needed. Since it's been long enough since the deprecation,
we've decided to fully remove support for RULE privilege,
so GRANT/REVOKE and privilege functions will no longer accept it.

Author: Fujii Masao
Reviewed-by: Nathan Bossart
Discussion: https://postgr.es/m/976a3581-6939-457f-b947-fc3dc836c083@oss.nttdata.com