Remove vacuum_defer_cleanup_age

Folks,

Given that hot_standby_feedback is pretty bulletproof now, and a lot of
the work in reducing replay conflicts, I think the utility of
vacuum_defer_cleanup_age is at an end.  I really meant so submit a patch
to remove it to 9.6, but it got away from me.

Any objections to removing the option in 10?

-- 
--
Josh Berkus
Red Hat OSAS
(any opinions are my own)