scsi: iscsi_tcp: Remove a set-but-not-used variable

This patch avoids that gcc reports the following warning when
building with W=1:

drivers/scsi/iscsi_tcp.c:166:24: warning: variable ?session? set but not used [-Wunused-but-set-variable]

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Cc: Lee Duncan <lduncan@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>