Revert 019_replslot_limit.pl related debugging aids.

This reverts most of 91c0570a79, f28bf667f6, fe0972ee5e, afdeff1052. The
only thing left is the retry loop in 019_replslot_limit.pl that avoids
spurious failures by retrying a couple times.

We haven't seen any hard evidence that this is caused by anything but slow
process shutdown. We did not find any cases where walsenders did not vanish
after waiting for longer. Therefore there's no reason for this debugging code
to remain.

Discussion: https://postgr.es/m/20220530190155.47wr3x2prdwyciah@alap3.anarazel.de
Backpatch: 15-

src/backend/storage/lmgr/lwlock.c

@@ -1942,10 +1942,3 @@ LWLockHeldByMeInMode(LWLock *l, LWLockMode mode)
 	}
 	return false;
 }
-
-/* temp debugging aid to analyze 019_replslot_limit failures */
-int
-LWLockHeldCount(void)
-{
-	return num_held_lwlocks;
-}