Fix #5: call Kbuild clean only where needed

The problem is both automake and kbuild recurse into subdirectories. In
`make distcheck`, automake cleans its `SUBDIRS` and only then tries to
clean the toplevel one. This in turn will trigger a kbuild clean that
will try to recurse into its own (now empty) subdirectories.

Avoid this issue by removing Kbuild clean from the toplevel.

Makefile.am

@@ -82,9 +82,6 @@ modules_install:
 	$(MAKE) -C "$(LINUX_SOURCE_DIR)" M="@abs_srcdir@" \
 		INSTALL_MOD_DIR="$(INSTALL_MOD_DIR)" modules_install
 
-clean-local:
-	$(MAKE) -C "$(LINUX_SOURCE_DIR)" M="@abs_srcdir@" clean
-
 mydist:
 	git -C $(srcdir) log > ChangeLog
 	@REV=`git -C $(srcdir) describe` && \