Add clear element to nuget.config (#10052)

Author: jviau

NuGet.config

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
+    <clear />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
     <add key="Microsoft.Azure.Functions.PowerShellWorker" value="https://azfunc.pkgs.visualstudio.com/e6a70c92-4128-439f-8012-382fe78d6396/_packaging/Microsoft.Azure.Functions.PowerShellWorker/nuget/v3/index.json" />
     <add key="AzureFunctions@internalrelease" value="https://azfunc.pkgs.visualstudio.com/e6a70c92-4128-439f-8012-382fe78d6396/_packaging/AzureFunctions%40internalrelease/nuget/v3/index.json" />