scsi: core: remove redundant assignment to shost->use_blk_mq

The first assignment to shost->use_blk_mq is redundant as it is
overwritten by the following statement. Remove this redundant code.

Detected by CoverityScan, CID#1466993 ("Unused value")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>