kbuild: remove unneeded '+' marker from cmd_clean

This '+' was added a long time ago:

| commit c23e6bf05f7802e92fd3da69a1ed35e56f9c85bb (HEAD)
| Author: Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
| Date:   Mon Oct 28 01:16:34 2002 -0600
|
|     kbuild: Fix a "make -j<N>" warning
|
| diff --git a/scripts/Makefile.clean b/scripts/Makefile.clean
| index 2c843e0380bc..e7c392fd5788 100644
| --- a/scripts/Makefile.clean
| +++ b/scripts/Makefile.clean
| @@ -42,7 +42,7 @@ quiet_cmd_clean = CLEAN   $(obj)
|
|  __clean: $(subdir-ymn)
|  ifneq ($(strip $(__clean-files) $(clean-rule)),)
| -        $(call cmd,clean)
| +        +$(call cmd,clean)
|  else
|          @:
|  endif

At that time, cmd_clean contained $(clean-rule), which was able to
invoke sub-make. That was why cleaning with the -j option showed:
warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.

It is not the case any more; cmd_clean now just runs the 'rm' command.
The '+' marker is pointless.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>