workqueue: remove spurious WARN_ON_ONCE(in_irq()) from try_to_grab_pending()

e0aecdd8 ("workqueue: use irqsafe timer for delayed_work") made
try_to_grab_pending() safe to use from irq context but forgot to
remove WARN_ON_ONCE(in_irq()).  Remove it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>