Cleanup

- drop unused using directive
- add a comment

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

shadowsocks-csharp/Encryption/EncryptorBase.cs

@@ -1,5 +1,4 @@
-using System.Security.Cryptography;
-using System.Text;
+using System.Text;
 
 namespace Shadowsocks.Encryption
 {

shadowsocks-csharp/Encryption/EncryptorFactory.cs

@@ -1,7 +1,7 @@
-
-using System;
+using System;
 using System.Collections.Generic;
 using System.Reflection;
+
 namespace Shadowsocks.Encryption
 {
     public static class EncryptorFactory

shadowsocks-csharp/Encryption/IEncryptor.cs

@@ -1,6 +1,4 @@
 using System;
-using System.Collections.Generic;
-using System.Text;
 
 namespace Shadowsocks.Encryption
 {

shadowsocks-csharp/Encryption/MbedTLSEncryptor.cs

@@ -1,9 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Runtime.InteropServices;
-using System.Security.Cryptography;
-using System.Text;
-using System.Threading;
 
 namespace Shadowsocks.Encryption
 {

shadowsocks-csharp/Encryption/SodiumEncryptor.cs

@@ -1,7 +1,5 @@
 using System;
 using System.Collections.Generic;
-using System.Text;
-using System.Threading;
 
 namespace Shadowsocks.Encryption
 {

shadowsocks-csharp/Util/SocketUtil.cs

@@ -52,6 +52,7 @@ namespace Shadowsocks.Util
 
             if (endPoint is DnsEndPoint)
             {
+                // use dual-mode socket
                 var socket = new Socket(AddressFamily.InterNetworkV6, socketType, protocolType);
                 socket.SetSocketOption(SocketOptionLevel.IPv6, (SocketOptionName)27, false);