Documentation/feature-removal-schedule.txt: remove ns_cgroup from feature-removal...

Documentation/feature-removal-schedule.txt: remove ns_cgroup from feature-removal-schedule.txt

Commit 6a2ec754347edf ("cgroup: remove the ns_cgroup") removed the
ns_cgroup but it forgot to remove the related doc in
feature-removal-schedule.txt.

Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Cc: Daniel Lezcano <daniel.lezcano@free.fr>
Cc: Serge E.  Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>