kbuild: remove {CLEAN,MRPROPER,DISTCLEAN}_DIRS

Merge {CLEAN,MRPROPER,DISTCLEAN}_DIRS into {CLEAN,MRPROPER,DISTCLEAN}_FILES
because the difference is just the -r option passed to the 'rm' command.

Do likewise as commit 1634f2bf

 ("kbuild: remove clean-dirs syntax").

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>