scsi: core: Remove 'current_tag'

The 'current_tag' field in struct scsi_device is unused now; remove it.

Link: https://lore.kernel.org/r/1631696835-136198-4-git-send-email-john.garry@huawei.com
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>