Remove duplicate package source entry in NuGet.config

Author: mmitche

NuGet.config

@@ -6,6 +6,7 @@
     <!--  Begin: Package sources from dotnet-dotnet -->
     <!--  End: Package sources from dotnet-dotnet -->
     <!--End: Package sources managed by Dependency Flow automation. Do not edit the sources above.-->
+    <add key="darc-pub-dotnet-dotnet-50d7970" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-dotnet-50d79701/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="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
   </packageSources>