drm/i915: Remove unused inline function drain_delayed_work()

It is not used since commit 058179e7 ("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