Remove unused macro to fix the following compilation warning:

drivers/target//iscsi/iscsi_target_stat.c:31: warning: macro "ISCSI_INST_LAST_FAILURE_TYPE" is not used [-Wunused-macros]
	#define ISCSI_INST_LAST_FAILURE_TYPE 0

Link: https://lore.kernel.org/r/20210228055645.22253-10-chaitanya.kulkarni@wdc.comReviewed-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>