csky: remove builtin-dtb Kbuild

Remove the builtin-dtb implementation in arch/csky.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>

arch/csky/boot/dts/Makefile

@@ -1,13 +1,6 @@
 dtstree	:= $(srctree)/$(src)
 
-ifneq '$(CONFIG_CSKY_BUILTIN_DTB)' '""'
-builtindtb-y := $(patsubst "%",%,$(CONFIG_CSKY_BUILTIN_DTB))
-dtb-y += $(builtindtb-y).dtb
-obj-y += $(builtindtb-y).dtb.o
-.SECONDARY: $(obj)/$(builtindtb-y).dtb.S
-else
 dtb-y := $(patsubst $(dtstree)/%.dts,%.dtb, $(wildcard $(dtstree)/*.dts))
-endif
 
 always += $(dtb-y)
 clean-files += *.dtb *.dtb.S