Subjct: Revert memory cgroup dirty_ratio Documentation.

The commit ece72400 adds documentation
for memcg's dirty ratio. But the function is not implemented yet.
Remove the documentation for avoiding confusing users.
Signed-off-by: NKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: NGreg Thelen <gthelen@google.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>