scsi: remove the disable_blk_mq host flag

We've had scsi-mq for 2.5 years now, so we can remove the unused flag to
disable the code on a per-host basis that was put in for unexpected
emergencies during bringup.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>