libiscsi: Remove set-but-not-used variables

Avoid that building with W=1 causes gcc to report warnings about
set-but-not-used variables.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>