Remove obsolete package sources from NuGet.config

Author: joperezr

NuGet.config

@@ -8,10 +8,7 @@
     <!--End: Package sources managed by Dependency Flow automation. Do not edit the sources above.-->
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
     <add key="dotnet-tools" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json" />
-    <add key="dotnet5" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet5/nuget/v3/index.json" />
-    <add key="dotnet3.1" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet3.1/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="nightly_iot_builds" value="https://pkgs.dev.azure.com/dotnet/IoT/_packaging/nightly_iot_builds/nuget/v3/index.json" />
   </packageSources>
   <disabledPackageSources />
 </configuration>