Clean nuget.config to meet the policy requirment

Author: xuzhg

sln/.nuget/NuGet.Config

@@ -6,9 +6,6 @@
   </solution>
   <packageSources>
     <clear />
-    <add key="nuget.org" value="https://www.nuget.org/api/v2/" />
-    <add key="buildTools" value="https://www.myget.org/F/30de4ee06dd54956a82013fa17a3accb/" />
-    <add key="externalComponentDependencies" value="https://www.myget.org/F/02a8fd0d231848d2ae32cd901e273000" />
-    <add key="ODataLibNightly" value="https://www.myget.org/F/odlnightly" />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
 </configuration>