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>

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)