Clean up unnecessary use of RefCell

src/main.rs

@@ -134,7 +134,7 @@ fn main() {
         .expect("failed to get initial audio devices and sessions");
 
     let (tx, rx): (Sender<String>, Receiver<String>) = mpsc::channel();
-    *WIN_CHANGE_CHANNEL_TX.lock().unwrap().borrow_mut() = Some(tx);
+    *WIN_CHANGE_CHANNEL_TX.lock().unwrap() = Some(tx);
 
     thread::spawn(move || {
         win_event_hook_loop();