Remove unused usings (#2217)

Source/MQTTnet.Tests/Server/Security_Tests.cs

@@ -2,9 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System.Collections.Generic;
-using System.Threading.Tasks;
-using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Adapter;
 using MQTTnet.Exceptions;
 using MQTTnet.Internal;