sql: should not allow AOST on crdb_internal

[tbg]

select * from crdb_internal.gossip_alerts AS OF SYSTEM TIME '2018-01-01';

works, but the AOST clause has no effect (and there's no possible world in which it could have a desired effect).

This doesn't seem particularly urgent to fix since crdb_internal is undocumented anyway.

cc @knz

Jira issue: CRDB-5719

[jordanlewis]

Hey @RaduBerinde I'm kicking this over to your team.

[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it in
5 days to keep the issue queue tidy. Thank you for your contribution
to CockroachDB!

[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it in
10 days to keep the issue queue tidy. Thank you for your contribution
to CockroachDB!