Don't display txid in toast

While we should create a success screen to display the Payjoin Success
with a link to mempool.space, I'm not comfortable linking directly from the
alert, since some people might not want to reveal their IP addres to
the explorer, and making a stopgap payjoin success screen seems like
an unnecessary hack when we plan to follow up with a proper payjoin history.

So this just gets rid of the txid that renders for now.

lib/_pkg/payjoin/event.dart

@@ -51,7 +51,7 @@ class _PayjoinEventListenerState extends State<PayjoinEventListener> {
       if (event is PayjoinBroadcastEvent) {
         showToastWidget(
           position: ToastPosition.top,
-          AlertUI(text: 'Payjoin transaction broadcast: ${event.txid}'),
+          const AlertUI(text: 'Payjoin transaction detected'),
           animationCurve: Curves.decelerate,
         );
       }