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/arm/mach-uniphier/Makefile

@@ -1 +0,0 @@
-obj- += dummy.o