drm/i915: Remove unused inline function drain_delayed_work()

It is not used since commit 058179e ("drm/i915/gt: Replace
hangcheck by heartbeats")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200715032104.32052-1-yuehaibing@huawei.com

Author: YueHaibing

drivers/gpu/drm/i915/i915_utils.h

@@ -266,19 +266,6 @@ static inline int list_is_last_rcu(const struct list_head *list,
 	return READ_ONCE(list->next) == head;
 }
 
-/*
- * Wait until the work is finally complete, even if it tries to postpone
- * by requeueing itself. Note, that if the worker never cancels itself,
- * we will spin forever.
- */
-static inline void drain_delayed_work(struct delayed_work *dw)
-{
-	do {
-		while (flush_delayed_work(dw))
-			;
-	} while (delayed_work_pending(dw));
-}
-
 static inline unsigned long msecs_to_jiffies_timeout(const unsigned int m)
 {
 	unsigned long j = msecs_to_jiffies(m);