scsi: advansys: remove unused variable 'srb_tag' in adv_isr_callback

drivers/scsi/advansys.c: In function 'adv_isr_callback':
drivers/scsi/advansys.c:5952:6: warning:
 variable 'srb_tag' set but not used [-Wunused-but-set-variable]

It never used since introduction in
commit 9c17c62a

 ("advansys: use shared host tag map for command lookup")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>