remove legacy nuget feed no longer reachable.

NuGet.Config

@@ -4,7 +4,6 @@
   <packageSources>
     <clear />
     <add key="api.nuget.org" value="https://api.nuget.org/v3/index.json" />    
-    <add key="dotnet-eng" value="https://nuget.avaloniaui.net/repository/avalonia-devdeps/index.json" protocolVersion="3" />
     <add key="skiasharp" value="https://aka.ms/skiasharp-eap/index.json" />
   </packageSources>
 </configuration>