Fixes: https://github.com/xamarin/xamarin-android/issues/7764

Context: d236af54
Context: https://github.com/xamarin/AndroidX/issues/64#issuecomment-609970584

.NET Android dropped support for `$(BundleAssemblies)`=True; see also
commit d236af54 and [a comment on xamarin/AndroidX#64][0]:

> After internal discussion, we are strongly considering removing
> support for `$(BundleAssemblies)` in .NET 5, as it increases
> on-device memory use and slows down app startup times.

However, this "dropping of support" was incomplete; setting it would
result in broken apps!

	% dotnet new android -n android-ba
	% cd android-ba
	% dotnet build -p:BundleAssemblies=true
	# run the app, or just look at the .apk contents
	% unzip -l bin/Debug/net7.0-android/*-Signed.apk | grep assem
	      660  02-03-2023 14:13   assemblies/rc.bin
	# Note: `assemblies/assemblies.blob` is not present, but is required!

Fix this by *removing* most usage of the `$(BundleAssemblies)` MSBuild
property as well as all the code affected by it.

  - Stop running bundle test apps on CI
  - Remove bundle unit tests
  - Remove the `CodeGen-MkBundle` project from the tests solution
  - Remove the `Xamarin.Android.MakeBundle-Tests` from the tests
    solution
  - Remove all native code (and configuration) related to mkbundle
  - Warn about `$(BundleAssemblies)` usage in .NET projects.
    A XA1035 warning will be emitted when `$(BundleAssemblies)`=True
    in .NET Android apps.

Preserve a couple of related properties in the `<BuildApk/>` and
`<GeneratePackageManagerJava/>` tasks, because they are still
used by the debugger targets from `xamarin/monodroid`.

[0]: https://github.com/xamarin/AndroidX/issues/64#issuecomment-6099