Don't block the script listener thread on window.close().

This does not appear to have any effect. I believe this has been the case
since e841065351, when the helper thread was
introduced.

It was added in a7ef1cd35e, where it blocked the
script thread rather than this helper thread.

components/compositing/compositor.rs

@@ -529,9 +529,8 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 return false
             }
 
-            (Msg::Exit(channel), _) => {
+            (Msg::Exit, _) => {
                 self.start_shutting_down();
-                let _ = channel.send(());
             }
 
             (Msg::ShutdownComplete, _) => {