Remove unused usings

Source/MQTTnet/Client/MqttClientOptionsBuilder.cs

@@ -1,7 +1,5 @@
 using System;
-using System.Collections.Generic;
 using System.Linq;
-using System.Net;
 using MQTTnet.Serializer;
 
 namespace MQTTnet.Client

Source/MQTTnet/Client/MqttClientOptionsBuilderWebSocketParameters.cs

@@ -1,7 +1,5 @@
-using System;
-using System.Collections.Generic;
+using System.Collections.Generic;
 using System.Net;
-using System.Text;
 
 namespace MQTTnet.Client
 {