cgroup: remove obsolete comment above struct cgroupstats

There's no flag in the delay accounting structure indicates that the task
is waiting on IO since commit 1193829d ("delayacct: cleanup flags in
struct task_delay_info and functions use it"). So remove the comment.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>