Return gst::BusSyncReply::Drop from the bus sync handler in the Rust sendrecv example

Otherwise all messages accumulate on the queue inside the bus and
nothing is ever removing them from there.

We handle messages elsewhere and only intercept them from the sync
handler.

webrtc/sendrecv/gst-rust/src/main.rs

@@ -819,7 +819,7 @@ fn main() {
                 let send_gst_msg_tx = Mutex::new(send_gst_msg_tx);
                 bus.set_sync_handler(move |_, msg| {
                     let _ = send_gst_msg_tx.lock().unwrap().try_send(msg.clone());
-                    gst::BusSyncReply::Pass
+                    gst::BusSyncReply::Drop
                 });
 
                 // Create our application control logic