Fixed duplicate class issue with new registrars

The class “ProgressLabel” was being compiled twice with the same native name using the new Xamarin.iOS registrars, resulting in a build error.

As a workaround I removed the linked file **ProgressLabel.cs** from the project and simply renamed the class. Of course the name doesn't have to stay, but due to the behavior of the new registrars, the project won't build without at least some change.

It may also be more appropriate to change the class within the Xamarin.Auth.iOS project rather than this one, or possibly even both. But in terms of just fixing the build error it's arbitrary.