Remove unused usings (#2217)

Source/MQTTnet.Tests/Server/Subscription_TopicHash_Tests.cs

@@ -1,10 +1,5 @@
-using System;
-using System.Collections.Generic;
 using System.Diagnostics;
-using System.Linq;
 using System.Text;
-using System.Threading.Tasks;
-using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Server;