removed unused usings

Author: Tom Gardham-Pallister

src/Rafty/Raft/IServersInCluster.cs

@@ -1,9 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
-using System.Security.Cryptography.X509Certificates;
-using System.Text;
-using Rafty.Infrastructure;
 using Rafty.ServiceDiscovery;
 
 namespace Rafty.Raft