Fixup feeds

Author: mmitche

NuGet.config

@@ -9,7 +9,6 @@
     <add key="dotnet-tools" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json" />
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
     <add key="vs-buildservices" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/vs-buildservices/nuget/v3/index.json" />
-    <add key="dotnet-core" value="https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json" />
     <add key="dotnet5-roslyn" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet5/nuget/v3/index.json" />
     <add key="mlnet-daily" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/MachineLearning/nuget/v3/index.json" />
     <add key="mlnet-testdata" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/machinelearning-testdata/nuget/v3/index.json" />