Make dotnet/wpf an assetless build

[ViktorHofer]

See dotnet/dotnet#711 for more context

WPF doesn't ship via its official build anymore and the YML should be converted to an assetless build. See https://github.com/dotnet/sourcelink/blob/ViktorHofer-patch-1/azure-pipelines.yml for an example on how to do this but keep OneLocBuild enabled.

@dotnet/wpf-developers