kthread_work: remove the unused wait_queue_head

The wait_queue_head_t kthread_work->done is unused since
flush_kthread_work() has been re-implemented.  Let's remove it
including the initialization code.  This makes
DEFINE_KTHREAD_WORK_ONSTACK() unnecessary, removed.

tj: Updated description.  Removed DEFINE_KTHREAD_WORK_ONSTACK().

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