Update nuget.config based on guidance (#2003)

Author: daxian-dbw

nuget.config

@@ -3,7 +3,8 @@
   <packageSources>
     <clear />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
-    <add key="powershell-core" value="https://powershell.myget.org/F/powershell-core/api/v3/index.json" />
-    <add key="powershell-modules" value="https://www.powershellgallery.com/api/v2/" />
   </packageSources>
+  <disabledPackageSources>
+    <clear />
+  </disabledPackageSources>
 </configuration>