nds32: remove unused BUILTIN_DTB from arch/nds32/Makefile

This is not used or exported.

BUILTIN_DTB is locally defined and used in arch/nds32/boot/dts/Makefile.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

arch/nds32/Makefile

@@ -31,12 +31,6 @@ core-y				+= arch/nds32/kernel/ arch/nds32/mm/
 core-$(CONFIG_FPU)              += arch/nds32/math-emu/
 libs-y				+= arch/nds32/lib/
 
-ifneq '$(CONFIG_NDS32_BUILTIN_DTB)' '""'
-BUILTIN_DTB := y
-else
-BUILTIN_DTB := n
-endif
-
 ifdef CONFIG_CPU_LITTLE_ENDIAN
 KBUILD_CFLAGS   += $(call cc-option, -EL)
 KBUILD_AFLAGS   += $(call cc-option, -EL)