Remove private reflection from DependencyModel

[ericstj]

Remove the following:
https://github.com/dotnet/core-setup/blob/1e298560b0dce474d2b0088350312442e8727190/src/managed/Microsoft.Extensions.DependencyModel/UnifiedJsonReader.Utf8JsonReader.cs#L149-L152

Once https://github.com/dotnet/corefx/issues/34768 is fixed

[eerhardt]

Triaged. This can't happen until the underlying Utf8JsonReader API is exposed. Moving to future.