scsi: remove scsi_next_command

There's only one caller left, so inline it and reduce the blk-mq vs !blk-mq
diff a litte bit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>