wscript: drop traces of shutdown() handling

autowaf has no real shutdown functionality anyway. The automatic
shutdown function that could have been called wouldn't work anyway, as
it takes an argument.

The only reason it doesn't fail is that the top level wscript has no
shutdown handling and doesn't recurse to other scripts, so it is all
dead code.

libs/surfaces/wscript

@@ -95,6 +95,3 @@ def build(bld):
         bld.recurse('maschine2')
     if bld.is_defined ('HAVE_WEBSOCKETS'):
         bld.recurse('websockets')
-
-def shutdown():
-    autowaf.shutdown()