scsi: target: core: Remove unused variable 'unit_serial_len'

Variable 'unit_serial_len' is just being assigned and it's never used
anywhere else. Remove it.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20221101105326.31037-1-colin.i.king@gmail.com


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