kbuild: remove empty rules for makefiles

The previous commit made 'MAKEFLAGS += -rR' effective in the top
Makefile regardless of O= option, GNU Make versions.

The top Makefile does not need to cancel implicit rules for makefiles.

There is still one place where an empty rule is useful. Since -rR is
effective only after sub-make, GNU Make would try implicit rules to
update the top Makefile. Although it is not a big overhead, cancel it
just in case.

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