powerpc/pseries: remove duplicate SCSI_BNX2_ISCSI in pseries_defconfig

Remove duplicate assignment of SCSI_BNX2_ISCSI in pseries_defconfig introduced by:
37e0c21e powerpc/pseries: Enable iSCSI support for a number of cards

causes warning:
arch/powerpc/configs/pseries_defconfig:151:warning: override: reassigning to symbol SCSI_BNX2_ISCSI

index c9f212b..80bc5de 100644
@@ -148,7 +148,6 @@ CONFIG_SCSI_SAS_ATTRS=m
 CONFIG_SCSI_CXGB3_ISCSI=m
 CONFIG_SCSI_CXGB4_ISCSI=m
 CONFIG_SCSI_BNX2_ISCSI=m
-CONFIG_SCSI_BNX2_ISCSI=m
 CONFIG_BE2ISCSI=m
 CONFIG_SCSI_IBMVSCSI=y
 CONFIG_SCSI_IBMVFC=m