Deleted instantiation "set :: (type) itself".

--- a/src/HOL/Typedef.thy	Wed May 07 10:56:49 2008 +0200
+++ b/src/HOL/Typedef.thy	Wed May 07 10:56:50 2008 +0200
@@ -160,15 +160,6 @@
 
 end
 
-instantiation "set" :: ("type") itself
-begin
-
-definition "itself = TYPE('a set)"
-
-instance ..
-
-end
-
 hide (open) const itself
 
 end