* Remove harvesting of M.E.DependencyModel

The netstandard1.6 configuration of Microsoft.Extensions.DependencyModel
isn't built anymore. Instead the already built matching binary from the
latest available package version is redistributed when packaging the
DependencyModel library.

Also dropping the netstandard1.3 asset and the net451 one as the
minimum supported set of platforms are ones that support netstandard2.0.

In addition to the harvesting removal, cleaning up the src project which
had an unnecessary condition and property set.

Contributes to https://github.com/dotnet/runtime/issues/47530