Remove IMvxViewModel<TResult> and IMvxViewModel<in TParameter, TResult>
## 🏗 Enhancement Proposal

<!--- A clear and concise description for your idea --->
Like the title states, I'd propose to remove `IMvxViewModel<TResult>` and `IMvxViewModel<in TParameter, TResult>`as they don't serve any value anymore.
https://github.com/MvvmCross/MvvmCross/blob/2bf57322efa3c9c5022f4cee65d8c3c89a1a7ba5/MvvmCross/ViewModels/IMvxViewModel.cs#L45-L54

### Pitch

<!--- How will this refactor improve contributors' lives? Input as many details as possible! --->
Since MvvmCross 9, the MvxNavigationService got its ability removed to return objects back to the parent viewmodel.
However, it seems like these interfaces are still leftovers that don't really make sense anymore now that you can't return data in that way.

### Platforms affected (mark all that apply)

(Assuming all platforms as it is in the core layer of Mvx)
- [x] :iphone: iOS
- [x] :robot: Android
- [x] :checkered_flag: WPF
- [x] :earth_americas: UWP
- [x] :apple: MacOS
- [x] :tv: tvOS
- [x] :monkey: Xamarin.Forms
