workqueue: Remove useless pool->dying_workers

A dying worker is first moved from pool->workers to pool->dying_workers
in set_worker_dying() and removed from pool->dying_workers in
detach_dying_workers().  The whole procedure is in the some lock context
of wq_pool_attach_mutex.

So pool->dying_workers is useless, just remove it and keep the dying
worker in pool->workers after set_worker_dying() and remove it in
detach_dying_workers() with wq_pool_attach_mutex held.

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