Gets rid of a couple of VS SLN Warnings - due to dupe'd using's decared (done a tidy up)

src/Umbraco.Core/Security/AuthenticationExtensions.cs

@@ -1,15 +1,10 @@
 using System;
 using System.Collections.Concurrent;
-using System.Collections.Concurrent;
-using System.Collections.Generic;
-using System.Globalization;
 using System.Globalization;
 using System.Linq;
 using System.Security.Claims;
 using System.Security.Principal;
-using System.Text;
 using System.Threading;
-using System.Threading.Tasks;
 
 namespace Umbraco.Core.Security
 {