nds32: remove unneeded clean-files for DTB

These patterns are cleaned-up by the top-level Makefile
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Greentime Hu <green.hu@gmail.com>
Signed-off-by: Greentime Hu <green.hu@gmail.com>

arch/nds32/boot/dts/Makefile

@@ -5,5 +5,3 @@ else
 BUILTIN_DTB :=
 endif
 obj-$(CONFIG_OF) += $(BUILTIN_DTB)
-
-clean-files := *.dtb *.dtb.S