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>

drivers/scsi/scsi_priv.h

@@ -29,7 +29,6 @@ extern int scsi_init_hosts(void);
 extern void scsi_exit_hosts(void);
 
 /* scsi.c */
-extern bool scsi_use_blk_mq;
 int scsi_init_sense_cache(struct Scsi_Host *shost);
 void scsi_init_command(struct scsi_device *dev, struct scsi_cmnd *cmd);
 #ifdef CONFIG_SCSI_LOGGING