scsi: core: Delete scsi_use_blk_mq

Module param scsi_use_blk_mq has not been referenced for some time, so zap
it.

Link: https://lore.kernel.org/r/1581355992-139274-1-git-send-email-john.garry@huawei.com


Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>