Removed some unused variables

Fixes issue #1989

Author: jstedfast

MailKit/Net/Imap/ImapCommand.cs

@@ -538,7 +538,6 @@ public bool Step ()
 		{
 			var supportsLiteralPlus = (Engine.Capabilities & ImapCapabilities.LiteralPlus) != 0;
 			var response = ImapCommandResponse.None;
-			var idle = UserData as ImapIdleContext;
 			ImapToken token;
 
 			// construct and write the command tag if this is the initial state
@@ -681,7 +680,6 @@ public async Task<bool> StepAsync ()
 		{
 			var supportsLiteralPlus = (Engine.Capabilities & ImapCapabilities.LiteralPlus) != 0;
 			var response = ImapCommandResponse.None;
-			var idle = UserData as ImapIdleContext;
 			ImapToken token;
 
 			// construct and write the command tag if this is the initial state

MailKit/Net/Pop3/Pop3Stream.cs

@@ -328,7 +328,6 @@ async Task<int> ReadAheadAsync (CancellationToken cancellationToken)
 			AlignReadAheadBuffer (out int start, out int end);
 
 			try {
-				var network = Stream as NetworkStream;
 				int nread;
 
 				cancellationToken.ThrowIfCancellationRequested ();