Remove dotnet8 feed comment (#939)

* Remove dotnet8 feed

This is no longer needed.

* add back feed

NuGet.config

@@ -9,7 +9,6 @@
     <!--End: Package sources managed by Dependency Flow automation. Do not edit the sources above.-->
     <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" />
-    <!-- Required as https://github.com/dotnet/runtime/pull/89509 did not get backported to .NET 8 Preview 7 -->
     <add key="dotnet8" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json" />
     <add key="dotnet-libraries" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-libraries/nuget/v3/index.json" />
   </packageSources>