Delete old stuff

This PR does some breaking change maintenance:

### Breaking Changes

* Legacy ReactiveUI 4.x commands are gone - pull them from source if you need them
* ObservableAsPropertyHelper is now no longer directly IObservable. Use `WhenAny` to get to these properties instead of grabbing the backing field.
* MemoizingMRUCache and ObservableAsyncMRUCache are both removed - the former has moved to Splat (add a `using Splat;` to fix this), and the latter is deprecated. Akavache is a better solution for ObservableAsyncMRUCache.