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>

samples/sockmap/Makefile

@@ -1,6 +1,3 @@
-# kbuild trick to avoid linker error. Can be omitted if a module is built.
-obj- := dummy.o
-
 # List of programs to build
 hostprogs-y := sockmap