cgroup/cpuset: Remove outdated comment in sched_partition_write()

The comment here is outdated and can cause confusion, from the code
perspective, there’s also no need for new comment, so just remove it.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>