pgsql: Still another try at stabilizing stats_ext test results.

Still another try at stabilizing stats_ext test results.

The stats_ext test is not expecting that autovacuum will touch
any of its tables; an expectation falsified by commit b07642dbc.
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 2dc16efed and 24566b359
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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/0936d1b6ffc6d59b4d17f3d6e53c036110c02b44

Modified Files
--------------
src/test/regress/expected/stats_ext.out | 54 +++++++++++++++++++-------------
src/test/regress/sql/stats_ext.sql | 55 ++++++++++++++++++++-------------
2 files changed, 65 insertions(+), 44 deletions(-)