removed bogus "error" message (it appeared during regular mode of operation)

--- a/src/Pure/Tools/simplifier_trace.scala	Wed Feb 26 23:09:29 2014 +0100
+++ b/src/Pure/Tools/simplifier_trace.scala	Thu Feb 27 11:41:32 2014 +0100
@@ -263,7 +263,6 @@
             case Some((id, _)) =>
               do_cancel(serial, id)
             case None =>
-              System.err.println("handle_cancel: unknown serial " + serial)
           }
 
         case Clear_Memory =>