kbuild: remove clean-dirs syntax

The only the difference between clean-files and clean-dirs is the -r
option passed to the 'rm' command.

You can always pass -r, and then remove the clean-dirs syntax.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
index 9fb2de9..e9ffdff 100644
--- a/Documentation/kbuild/makefiles.rst
+++ b/Documentation/kbuild/makefiles.rst
@@ -750,7 +750,8 @@
 generated by kbuild are deleted all over the kernel src tree when
 "make clean" is executed.
 
-Additional files can be specified in kbuild makefiles by use of $(clean-files).
+Additional files or directories can be specified in kbuild makefiles by use of
+$(clean-files).
 
 	Example::
 
@@ -761,17 +762,8 @@
 Kbuild will assume files to be in the same relative directory as the
 Makefile, except if prefixed with $(objtree).
 
-To delete a directory hierarchy use:
-
-	Example::
-
-		#scripts/package/Makefile
-		clean-dirs := $(objtree)/debian/
-
-This will delete the directory debian in the toplevel directory, including all
-subdirectories.
-
-To exclude certain files from make clean, use the $(no-clean-files) variable.
+To exclude certain files or directories from make clean, use the
+$(no-clean-files) variable.
 
 Usually kbuild descends down in subdirectories due to "obj-* := dir/",
 but in the architecture makefiles where the kbuild infrastructure