Retire pg_standby.

pg_standby was useful more than a decade ago, but now it is obsolete.
It has been proposed that we retire it many times.  Now seems like a
good time to finally do it, because "waiting restore commands"
are incompatible with a proposed recovery prefetching feature.

Discussion: https://postgr.es/m/20201029024412.GP5380%40telsasoft.com
Author: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Fujii Masao <masao.fujii@oss.nttdata.com>

contrib/Makefile

@@ -33,7 +33,6 @@ SUBDIRS = \
 		pg_buffercache	\
 		pg_freespacemap \
 		pg_prewarm	\
-		pg_standby	\
 		pg_stat_statements \
 		pg_surgery	\
 		pg_trgm		\