This reverts much of commit f03a9ca4,
but leaves the relpages/reltuples probe in select_parallel.sql.
The pg_stat_all_tables probes are unstable enough to be annoying,
and it no longer seems likely that they will teach us anything more
about the underlying problem.  I'd still like some more confirmation
though that the observed plan instability is caused by VACUUM leaving
relpages/reltuples as zero for one of these tables.

Discussion: https://postgr.es/m/CA+hUKG+0CxrKRWRMf5ymN3gm+BECHna2B-q1w8onKBep4HasUw@mail.gmail.com