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clearly outdated.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-

src/backend/commands/subscriptioncmds.c

@@ -1578,15 +1578,9 @@ DropSubscription(DropSubscriptionStmt *stmt, bool isTopLevel)
 
 	/*
 	 * Tell the cumulative stats system that the subscription is getting
-	 * dropped. We can safely report dropping the subscription statistics here
-	 * if the subscription is associated with a replication slot since we
-	 * cannot run DROP SUBSCRIPTION inside a transaction block.  Subscription
-	 * statistics will be removed later by (auto)vacuum either if it's not
-	 * associated with a replication slot or if the message for dropping the
-	 * subscription gets lost.
+	 * dropped.
 	 */
-	if (slotname)
-		pgstat_drop_subscription(subid);
+	pgstat_drop_subscription(subid);
 
 	table_close(rel, NoLock);
 }