revert "mm: memcontrol: fix possible css ref leak on oom"

Revert commit 1383399d ("mm: memcontrol: fix possible css ref leak
on oom").  Johannes points out "There is a task_in_memcg_oom() check
before calling mem_cgroup_oom()".

Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>