mm: remove unnecessary description about a non-exist gfp flag

Since __GFP_NOACCOUNT was removed by commit 20b5c303 ("Revert 'gfp:
add __GFP_NOACCOUNT'"), its description is not necessary.

Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>