Remove visibleForTesting annotation from ChangeNotifier.deliverChanges. (#55)

It is used in `lib/src/observable.dart`.
diff --git a/lib/src/change_notifier.dart b/lib/src/change_notifier.dart
index 82d4e51..9464740 100644
--- a/lib/src/change_notifier.dart
+++ b/lib/src/change_notifier.dart
@@ -49,7 +49,6 @@
   ///
   /// Returns `true` if changes were emitted.
   @override
-  @visibleForTesting
   @mustCallSuper
   bool deliverChanges() {
     List<ChangeRecord> changes;