Remove the "snapshot too old" feature.

Remove the old_snapshot_threshold setting and mechanism for producing
the error "snapshot too old", originally added by commit 848ef42b.
Unfortunately it had a number of known problems in terms of correctness
and performance, mostly reported by Andres in the course of his work on
snapshot scalability.  We agreed to remove it, after a long period
without an active plan to fix it.

This is certainly a desirable feature, and someone might propose a new
or improved implementation in the future.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CACG%3DezYV%2BEvO135fLRdVn-ZusfVsTY6cH1OZqWtezuEYH6ciQA%40mail.gmail.com
Discussion: https://postgr.es/m/20200401064008.qob7bfnnbu4w5cw4%40alap3.anarazel.de
Discussion: https://postgr.es/m/CA%2BTgmoY%3Daqf0zjTD%2B3dUWYkgMiNDegDLFjo%2B6ze%3DWtpik%2B3XqA%40mail.gmail.com

src/backend/utils/misc/guc_tables.c

@@ -3282,17 +3282,6 @@ struct config_int ConfigureNamesInt[] =
 		check_autovacuum_work_mem, NULL, NULL
 	},
 
-	{
-		{"old_snapshot_threshold", PGC_POSTMASTER, RESOURCES_ASYNCHRONOUS,
-			gettext_noop("Time before a snapshot is too old to read pages changed after the snapshot was taken."),
-			gettext_noop("A value of -1 disables this feature."),
-			GUC_UNIT_MIN
-		},
-		&old_snapshot_threshold,
-		-1, -1, MINS_PER_HOUR * HOURS_PER_DAY * 60,
-		NULL, NULL, NULL
-	},
-
 	{
 		{"tcp_keepalives_idle", PGC_USERSET, CONN_AUTH_TCP,
 			gettext_noop("Time between issuing TCP keepalives."),