Removed unused import.

backend/tests/Squidex.Web.Tests/Pipeline/CachingFilterTests.cs

@@ -8,7 +8,6 @@
 using System;
 using System.Collections.Generic;
 using System.Security.Claims;
-using System.Threading;
 using System.Threading.Tasks;
 using FakeItEasy;
 using Microsoft.AspNetCore.Http;