scsi: target: iscsi: Remove unused macro TEXT_LEN

scsi: target: iscsi: Remove unused macro TEXT_LEN

Remove unused macro to fix the following compilation warning:

drivers/target//iscsi/iscsi_target_nego.c:31: warning: macro "TEXT_LEN" is not used [-Wunused-macros]
	#define TEXT_LEN 4096

Link: https://lore.kernel.org/r/20210228055645.22253-11-chaitanya.kulkarni@wdc.com
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>