removed dead code;

--- a/src/Pure/Tools/simplifier_trace.ML	Wed Feb 19 15:57:02 2014 +0000
+++ b/src/Pure/Tools/simplifier_trace.ML	Wed Feb 19 20:07:31 2014 +0100
@@ -140,7 +140,6 @@
 fun output_result (id, data) =
   Output.result (Markup.serial_properties id) data
 
-val serialN = "serial"
 val parentN = "parent"
 val textN = "text"
 val memoryN = "memory"