cpuset: remove is_prs_invalid helper

The is_prs_invalid helper function is redundant as it serves a similar
purpose to is_partition_invalid. It can be fully replaced by the existing
is_partition_invalid function, so this patch removes the is_prs_invalid
helper.

Signed-off-by: Chen Ridong <chenridong@huawei.com>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>