kbuild: do not descend to ./Kbuild when cleaning

'make clean' descends into ./Kbuild, but does not clean anything
since everything is added to no-clean-files.

There is no need to descend to ./Kbuild in the first place.
We can drop the no-clean-files assignment.

With this, there is no more user of no-clean-files. I will keep it
for a while to see whether a new user will appear.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Documentation/kbuild/makefiles.rst

@@ -787,12 +787,6 @@ 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.
-This is only a special case used in the top level Kbuild file:
-
-	Example::
-
-		#Kbuild
-		no-clean-files := $(bounds-file) $(offsets-file)
 
 Usually kbuild descends down in subdirectories due to "obj-* := dir/",
 but in the architecture makefiles where the kbuild infrastructure

Kbuild

@@ -57,6 +57,3 @@ quiet_cmd_atomics = CALL    $<
 
 old-atomics: scripts/atomic/check-atomics.sh FORCE
 	$(call cmd,atomics)
-
-# Keep these three files during make clean
-no-clean-files := $(bounds-file) $(offsets-file) $(timeconst-file)

Makefile

@@ -1396,7 +1396,7 @@ DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS
 #
 clean: rm-dirs  := $(CLEAN_DIRS)
 clean: rm-files := $(CLEAN_FILES)
-clean-dirs      := $(addprefix _clean_, . $(vmlinux-alldirs))
+clean-dirs      := $(addprefix _clean_, $(vmlinux-alldirs))
 
 PHONY += $(clean-dirs) clean archclean vmlinuxclean
 $(clean-dirs):