removed dead code (see also db4bf4fb5492);

--- a/src/Pure/System/system_channel.scala	Wed Oct 16 13:02:35 2013 +0200
+++ b/src/Pure/System/system_channel.scala	Mon Oct 14 19:50:36 2013 +0200
@@ -15,7 +15,7 @@
 
 object System_Channel
 {
-  def apply(use_socket: Boolean = false): System_Channel =
+  def apply(): System_Channel =
     if (Platform.is_windows) new Socket_Channel else new Fifo_Channel
 }