dotnet-svcutil: Evaluate whether RemoveExtensibleDataObjectImpl is still needed

[imcarolwang]

RemoveExtensibleDataObjectImpl.cs removes generated IExtensibleDataObject/ExtensibleDataObject implementation from proxy types.

This looks like legacy behavior from early .NET Core versions (when IExtensibleDataObject support may have been incomplete), but IExtensibleDataObject has been available since .NET Core 2.0+. There also doesn’t appear to be an equivalent removal step in .NET Framework codegen.