dropped obsolete code equation for Id

--- a/src/HOL/Relation.thy	Sat Dec 24 15:53:09 2011 +0100
+++ b/src/HOL/Relation.thy	Sat Dec 24 15:53:09 2011 +0100
@@ -132,7 +132,7 @@
 lemma Id_on_iff: "((x, y) : Id_on A) = (x = y & x : A)"
 by blast
 
-lemma Id_on_def' [nitpick_unfold, code]:
+lemma Id_on_def' [nitpick_unfold]:
   "Id_on {x. A x} = Collect (\<lambda>(x, y). x = y \<and> A x)"
 by auto