mm/vmscan: remove page_queue_congested() comment

Commit 084f71ae5c(kill page_queue_congested()) removed
page_queue_congested().  Remove the page_queue_congested() comment in
vmscan pageout() too.

Signed-off-by: Vincent Li <macli@brc.ubc.ca>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>