dropped superfluous declaration attribute

--- a/src/HOL/Relation.thy	Wed Mar 13 13:46:16 2019 +0100
+++ b/src/HOL/Relation.thy	Sun Mar 10 15:16:45 2019 +0000
@@ -523,7 +523,7 @@
 subsubsection \<open>The identity relation\<close>
 
 definition Id :: "'a rel"
-  where [code del]: "Id = {p. \<exists>x. p = (x, x)}"
+  where "Id = {p. \<exists>x. p = (x, x)}"
 
 lemma IdI [intro]: "(a, a) \<in> Id"
   by (simp add: Id_def)