Remove nuget internal feed that was used for internal testing but no longer needed

Author: nagilson

NuGet.config

@@ -32,7 +32,6 @@
     <add key="dotnet7" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet7/nuget/v3/index.json" />
     <add key="dotnet7-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet7-transport/nuget/v3/index.json" />
     <add key="dotnet8" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json" />
-    <add key="dotnet8-internal" value="https://pkgs.dev.azure.com/dnceng/internal/_packaging/dotnet8-internal/nuget/v3/index.json" />
     <add key="dotnet8-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8-transport/nuget/v3/index.json" />
     <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />