Remove AutoJoinScriptTask.

I don't believe there is a case where it would make sense to drop the
ScriptReflow struct without joining the script thread. This approach should
be somewhat more robust, and avoids the code smell of a RAII guard in an
otherwise unused variable.

components/script/layout_interface.rs

@@ -179,6 +179,12 @@ pub struct ScriptReflow {
     pub query_type: ReflowQueryType,
 }
 
+impl Drop for ScriptReflow {
+    fn drop(&mut self) {
+        self.script_join_chan.send(()).unwrap();
+    }
+}
+
 /// Encapsulates a channel to the layout task.
 #[derive(Clone)]
 pub struct LayoutChan(pub Sender<Msg>);