Remove extra feed (microsoft#3661)

Author: vnbaaij

NuGet.config

@@ -3,12 +3,8 @@
   <packageSources>
     <clear />
     <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
-    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
   <packageSourceMapping>
-    <packageSource key="nuget.org">
-      <package pattern="Microsoft.AspNetCore.App.Internal.Assets" />
-    </packageSource>
     <packageSource key="dotnet-public">
       <package pattern="*" />
     </packageSource>