delete unnecessary usings

Author: antonfirsov

snippets/csharp/System.Net.Http/SocketsHttpHandler/ConnectCallback/program.cs

@@ -1,9 +1,7 @@
 using System;
-using System.IO;
 using System.Net;
 using System.Net.Http;
 using System.Net.Sockets;
-using System.Threading;
 using System.Threading.Tasks;
 
 class HttpClientHandler_SecureExample