Remove feeds that are no longer needed.

src/nuget.config

@@ -7,7 +7,5 @@
     <add key="api.nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
     <add key="myget.org/F/aarnott" value="https://www.myget.org/F/aarnott/api/v3/index.json" protocolVersion="3" />
     <add key="vs-devcore" value="https://www.myget.org/F/vs-devcore/api/v3/index.json" />
-    <add key="vs-validation" value="https://ci.appveyor.com/nuget/vs-validation" />
-    <add key="vs-threading" value="https://ci.appveyor.com/nuget/vs-threading" />
   </packageSources>
 </configuration>