workqueue: Remove deprecated __cancel_delayed_work()

__cancel_delayed_work() was deprecated by 136b5721 ("workqueue:
deprecate __cancel_delayed_work()") as cancel_delayed_work() was
updated so that it could be used from all contexts.  Enough time has
passed since the deprecation.  Let's remove it.

tj: description update

Signed-off-by: Tan Xiaojun <tanxiaojun@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>