Code cleanup: simplify NarratorNotifier (#646)

Now that the RaiseNotificationEvent API is available on all platforms where the Calculator app runs, we can remove the factory classes which switched between RaiseNotificationEvent and an alternative implementation based on live regions.

src/Calculator/Converters/BooleanToVisibilityConverter.cpp

@@ -1,11 +1,9 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
 #include "pch.h"
 #include "BooleanToVisibilityConverter.h"
 
-using namespace CalculatorApp::Common;
-
 using namespace Platform;
 using namespace Windows::Foundation;
 using namespace Windows::UI::Xaml;