The stats_ext test is not expecting that autovacuum will touch
any of its tables; an expectation falsified by commit b07642db.
Although I'm suspicious that there's something else going on that
makes extended stats estimates not 100% reproducible, it's pretty
easy to demonstrate that there are places in this test that fail
if an autovacuum updates the table's stats unexpectedly.

Hence, revert the band-aid changes made by 2dc16efe and 24566b35
in favor of summarily disabling autovacuum for all the tables that
this test checks estimated rowcounts for.

Also remove an evidently obsolete comment at the head of the test.

Discussion: https://postgr.es/m/15012.1585623298@sss.pgh.pa.us