pgstat: drop subscription stats without slot as well, fix comment

pgstat: drop subscription stats without slot as well, fix comment

There's no reason anymore to only drop subscription stats if associated with a
slot, now that stats drops are transactional. And since there's now no other
cleanup of stats, this would lead to stats for slot-less subscriptions to get
leaked (however most slot-less subs won't have stats).

Additionally, the comment referring to autovacuum cleaning up stats was
clearly outdated.

Author: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/CAD21AoAwiby3HeJE7vJe16Gr75RFfJ640dyHqvsiUhyKJTXPtw@mail.gmail.com
Backpatch: 15-