Remove unused import.

src/Renci.SshNet/Security/KeyExchangeECCurve25519.cs

@@ -1,5 +1,4 @@
-using System;
-using Renci.SshNet.Abstractions;
+using Renci.SshNet.Abstractions;
 using Renci.SshNet.Common;
 using Renci.SshNet.Messages.Transport;
 using Renci.SshNet.Security.Chaos.NaCl;