Remove a spinlock which prevents schedule_delayed_work() and
mod_delayed_work() from executing concurrently.

This was required back when mod_delayed_work() did not exist,
and had to be implemented with a cancel + schedule. See
commit e7c2f967 ("workqueue: use mod_delayed_work() instead of
__cancel + queue")

schedule_delayed_work() and mod_delayed_work() can now be used
concurrently. So the spinlock is no longer needed.
Signed-off-by: NSven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>