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/arc/config.mk b/arch/arc/config.mk
index a3b8df7..e408800 100644
--- a/arch/arc/config.mk
+++ b/arch/arc/config.mk
@@ -21,7 +21,7 @@
 CROSS_COMPILE := $(ARC_CROSS_COMPILE)
 endif
 
-PLATFORM_CPPFLAGS += -ffixed-r25 -D__ARC__ -DCONFIG_ARC -gdwarf-2
+PLATFORM_CPPFLAGS += -ffixed-r25 -D__ARC__ -gdwarf-2
 
 # Needed for relocation
 LDFLAGS_FINAL += -pie