cgroup: remove unneeded checks

"descendants" and "depth" are declared as int, so they can't be larger
than INT_MAX.  Static checkers complain and it's slightly confusing for
humans as well so let's just remove these conditions.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>