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 1634f2bfdb ("kbuild: remove clean-dirs syntax").
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>