Remove unused usings (#2217)

Source/MQTTnet.Tests/Formatter/MqttPacketSerialization_V5_Tests.cs

@@ -2,9 +2,7 @@
 // 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;
 using System.Buffers;
-using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Formatter;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;