kbuild: remove all dummy assignments to obj-

Now kbuild core scripts create empty built-in.o where necessary.
Remove "obj- := dummy.o" tricks.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

arch/mips/boot/dts/xilfpga/Makefile

@@ -2,6 +2,3 @@
 dtb-$(CONFIG_FIT_IMAGE_FDT_XILFPGA)	+= nexys4ddr.dtb
 
 obj-y				+= $(patsubst %.dtb, %.dtb.o, $(dtb-y))
-
-# Force kbuild to make empty built-in.o if necessary
-obj-				+= dummy.o