Remove myget feed
Remove myget feed

We haven't needed this for ages. While we could remove NuGet.config
entirely, it's probably worth keeping just like global.json, to make
the build less dependent on external factors.

jskeet/gcloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet,jskeet/google-cloud-dotnet,iantalarico/google-cloud-dotnet,iantalarico/google-cloud-dotnet,jskeet/google-cloud-dotnet,evildour/google-cloud-dotnet,googleapis/google-cloud-dotnet,jskeet/google-cloud-dotnet,evildour/google-cloud-dotnet,iantalarico/google-cloud-dotnet,evildour/google-cloud-dotnet,googleapis/google-cloud-dotnet