Retire pg_standby.

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pg_standby was useful more than a decade ago, but now it is obsolete.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 <[email protected]>
Reviewed-by: Heikki Linnakangas <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Reviewed-by: Fujii Masao <[email protected]>