scsi: default to scsi-mq

Remove the SCSI_MQ_DEFAULT config option and default to the blk-mq I/O
path now that we had plenty of testing, and have I/O schedulers for
blk-mq.  The module option to disable the blk-mq path is kept around for
now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>