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>

kernel/workqueue.c

@@ -1070,8 +1070,6 @@ static int try_to_grab_pending(struct work_struct *work, bool is_dwork,
 {
 	struct global_cwq *gcwq;
 
-	WARN_ON_ONCE(in_irq());
-
 	local_irq_save(*flags);
 
 	/* try to steal the timer if it exists */