Drop PrismApplication

[dansiegel]

Description

The Application really isn't needed by Prism with .NET MAUI. Everything that Prism made use of the Application for in Xamarin.Forms is either done with the App Builder or we have hooks in the Window which leaves the Application completely untouched. As a result this should be dropped completely. Any apps migrating will already need to be migrating their registration code to the App Builder there really isn't a reason to keep the PrismApplication class.