[build] remove Xamarin.Android.CSharp.targets (#9609)

Remove `Xamarin.Android.CSharp.targets` as this file is only used in
Xamarin.Android projects. SDK-style projects rely on "magical" imports
when you have `$(TargetPlatformIdentifier)=android`, and so projects
do not ever import this file directly.

Removing this file led me to find other files we can delete:

* `build-tools\scripts\MonoAndroidFramework.props` imported by some
  Xamarin.Android projects in the repo.

* `samples\VSAndroidApp` unused Xamarin.Android sample

* `tests\Mono.Android-Tests\Runtime-AppBundle` unused Xamarin.Android
  test project

* `tests\Mono.Android-Tests\Runtime-MultiDex` unused Xamarin.Android
  test project

I also removed any Xamarin.Android `.csproj` file, where I found a
".NET" equivalent. We could likely remove even more in the future, but
we'd want to check if the code or tests could be useful.

Author: jonathanpeppers