mm/oom_kill.c: fix badness() kerneldoc

Paramter @mem has been removed since v2.6.26, now delete it's comment.

Signed-off-by: Qinghuang Feng <qhfeng.kernel@gmail.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>