workqueue: Remove the unneeded cpumask empty check in wq_calc_pod_cpumask()

The cpumask empty check in wq_calc_pod_cpumask() has long been useless.
It just works purely as documents which states that the cpumask is not
possible empty after the function returns.

Now the code above is even more explicit that the cpumask is not empty,
so the document-only empty check can be removed.

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Tejun Heo <tj@kernel.org>