Unify functionality GenerateReferenceAssemblySource and GenerateReferenceSource

[ericstj]

GenAPI defines GenerateReferenceAssemblySource https://github.com/dotnet/arcade/blob/55c0d49b9bec66981f7dd6d8f51a8dad43e8fbe0/src/Microsoft.DotNet.GenAPI/build/Microsoft.DotNet.GenAPI.targets#L31-L47, but we don't use it in CoreFx.

Instead we use GenerateReferenceSource https://github.com/dotnet/corefx/blob/4a84b8798dbd861675014b420e4c01f6a2116125/Directory.Build.targets#L176-L195.

We should delete GenerateReferenceSource and make sure that GenerateReferenceAssemblySource can handle our scenarios, then update the docs to to use it (eg: by setting the property).

/cc @wtgodbe

[ericstj]

Make sure we address dotnet/corefx#31193 when doing this.