block: remove redundant passthrough check in blk_mq_need_time_stamp()

Simply checking the rq_flags is enough to determine if accounting is
being done for this request.

Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>