Revert "Accept relations of any kind in LOCK TABLE".

Revert "Accept relations of any kind in LOCK TABLE".

Revert 59ab4ac32, as well as the followup fix 33862cb9c, in all
branches.  We need to think a bit harder about what the behavior
of LOCK TABLE on views should be, and there's no time for that
before next week's releases.  We'll take another crack at this
later.

Discussion: https://postgr.es/m/16703-e348f58aab3cf6cc@postgresql.org