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

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>