Remove BroadcastChannel code from example.dart

Removed BroadcastChannel implementation from example.dart.

Author: terrier989

example/example.dart

@@ -16,9 +16,4 @@ void main() {
   // Do a CSS query
   print(divElement.querySelector('div > .greeting')!.text);
   // --> Hello world
-
-  final broadcastChannel = BroadcastChannel('Channel_Name');
-  broadcastChannel.onMessage.listen((event) {
-    print(event);
-  });
 }