Remove non-functional APIs that always return null, like Window.Current

[andrewleader]

This is the follow-on task from #1660. There are several APIs in our API surface that always return null and confuse developers, like Window.Current and also Window.Dispatcher and more. The presence of these non-functional APIs makes it more difficult to code and more difficult to migrate from UWP, since any developer would expect these APIs to work since they're there, but they actually just return null.

These APIs only worked in UWP, which was why we had them, but we need to clean up the API surface.

Expected behavior
We'll mark these APIs as deprecated with #1660, this issue tracks removing these APIs in 2.0.

[jaigak]

Instead of removing Window.Current, why not change it to return the currently focused window from the application on the screen regardless of the calling thread? In UWP, each window was in a separate thread (except AppWindow preview) however Window.Current is still needed in WinUI 3 so that you don't always need to directly have a reference to it.