removing duplicate code

--- a/src/HOL/Tools/Quickcheck/exhaustive_generators.ML	Fri Apr 08 16:31:14 2011 +0200
+++ b/src/HOL/Tools/Quickcheck/exhaustive_generators.ML	Fri Apr 08 16:31:14 2011 +0200
@@ -60,8 +60,6 @@
   end
   | mk_sumcases _ f T = f T
 
-fun mk_undefined T = Const(@{const_name undefined}, T)
-
 (** abstract syntax **)
 
 fun termifyT T = HOLogic.mk_prodT (T, @{typ "unit => Code_Evaluation.term"});