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>

usr/include/Makefile

@@ -115,6 +115,4 @@ header-test-y += $(filter-out $(header-test-), \
 			$(patsubst $(obj)/%,%, $(wildcard \
 			$(addprefix $(obj)/, *.h */*.h */*/*.h */*/*/*.h))))
 
-# For GNU Make <= 4.2.1, $(wildcard $(obj)/*/) matches to not only directories
-# but also regular files. Use $(filter %/, ...) just in case.
-clean-dirs += $(patsubst $(obj)/%/,%,$(filter %/, $(wildcard $(obj)/*/)))
+clean-files += $(filter-out Makefile, $(notdir $(wildcard $(obj)/*)))