The remnants of the code to support GC configs in which HandlePromotionFailure
might be disabled should be removed since it appears to serve no useful purpose.

As ###@###.### has pointed out only single-threaded serial gc
can currently provably avoid promotion failure, and as everyone seems to agree the
ability to turn it off appears to provide no real advantage (except for
switching off the paths that unwind from a failed scavenge because of
promotion failure, paths that have by now been tested thoroughly).

As a result it appears to make sense to remove support for that flag
and eliminate code that expects to run in a configuration where
promotion failure handling is disabled.

For the history buffs, promotion failure handling became the default
for all of HotSpot's collectors by 6.0 (and much earlier for some of the
collectors).