[RTDB] [Sample App] Added a delete option to items in the RTDB sample (#2411)

* Added delete option to RTDB sample

Merging since this does not impact on the CI breakage.

Author: PaulTR

packages/firebase_database/example/lib/main.dart

@@ -156,7 +156,16 @@ class _MyHomePageState extends State<MyHomePage> {
                   Animation<double> animation, int index) {
                 return SizeTransition(
                   sizeFactor: animation,
-                  child: Text("$index: ${snapshot.value.toString()}"),
+                  child: ListTile(
+                    trailing: IconButton(
+                      onPressed: () =>
+                          _messagesRef.child(snapshot.key).remove(),
+                      icon: Icon(Icons.delete),
+                    ),
+                    title: Text(
+                      "$index: ${snapshot.value.toString()}",
+                    ),
+                  ),
                 );
               },
             ),