cgroup/cpuset: drop useless cpumask_empty() in compute_effective_exclusive_cpumask()

Empty cpumasks can't intersect with any others. Therefore, testing for
non-emptyness is useless.

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>