kconfig: delete redundant CONFIG_${ARCH} definition

CONFIG_${ARCH} is defined by Kconfig.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/nios2/config.mk b/arch/nios2/config.mk
index 65a5a40..82bd887 100644
--- a/arch/nios2/config.mk
+++ b/arch/nios2/config.mk
@@ -12,7 +12,7 @@
 
 CONFIG_STANDALONE_LOAD_ADDR ?= 0x02000000
 
-PLATFORM_CPPFLAGS += -DCONFIG_NIOS2 -D__NIOS2__
+PLATFORM_CPPFLAGS += -D__NIOS2__
 PLATFORM_CPPFLAGS += -G0
 
 LDFLAGS_FINAL += --gc-sections