scsi: arcmsr: Remove set but not used variables 'id, lun'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/scsi/arcmsr/arcmsr_hba.c: In function 'arcmsr_drain_donequeue':
drivers/scsi/arcmsr/arcmsr_hba.c:1320:10: warning:
 variable 'lun' set but not used [-Wunused-but-set-variable]

drivers/scsi/arcmsr/arcmsr_hba.c:1320:6: warning:
 variable 'id' set but not used [-Wunused-but-set-variable]

Never used since introduction in commit ae52e7f0 ("arcmsr: Support 1024 scatter-gather list entries and improve AP while FW trapped and behaviors of EHs").

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