blk-mq: remove unnecessary blk_clear_rq_complete()

This patch removes two unnecessary blk_clear_rq_complete(),
the REQ_ATOM_COMPLETE flag is cleared inside blk_mq_start_request(),
so:

	- The blk_clear_rq_complete() in blk_flush_restore_request()
	needn't because the request will be freed later, and clearing
	it here may open a small race window with timeout.

	- The blk_clear_rq_complete() in blk_mq_requeue_request() isn't
	necessary too, even though REQ_ATOM_STARTED is cleared in
	__blk_mq_requeue_request(), in theory it still may cause a small
	race window with timeout since the two clear_bit() may be
	reordered.

Signed-off-by: Ming Lei <ming.lei@canoical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>