oom-kill: remove boost_dying_task_prio()

This is an almost-revert of commit 93b43fa5 ("oom: give the dying task a
higher priority").

That commit dramatically improved oom killer logic when a fork-bomb
occurs.  But I've found that it has nasty corner case.  Now cpu cgroup has
strange default RT runtime.  It's 0!  That said, if a process under cpu
cgroup promote RT scheduling class, the process never run at all.

If an admin inserts a !RT process into a cpu cgroup by setting
rtruntime=0, usually it runs perfectly because a !RT task isn't affected
by the rtruntime knob.  But if it promotes an RT task via an explicit
setscheduler() syscall or an OOM, the task can't run at all.  In short,
the oom killer doesn't work at all if admins are using cpu cgroup and don't
touch the rtruntime knob.

Eventually, kernel may hang up when oom kill occur.  I and the original
author Luis agreed to disable this logic.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Luis Claudio R. Goncalves <lclaudio@uudg.org>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>