workqueue: remove unused work_clear_pending()

In 8930caba ("workqueue: disable irq while manipulating PENDING"),
setting last CPU and clearing PENDING got merged into a single
operation (set_work_cpu_and_clear_pending()), which resulted that the
internal routine work_clear_pending() is not used any more.

tj: Minor description tweak.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>