removed unique ids -- now in session.scala;

--- a/src/Pure/System/isabelle_system.scala	Fri Dec 11 20:44:33 2009 +0100
+++ b/src/Pure/System/isabelle_system.scala	Fri Dec 11 22:31:24 2009 +0100
@@ -48,13 +48,6 @@
 
 class Isabelle_System
 {
-  /** unique ids **/
-
-  private var id_count: BigInt = 0
-  def id(): String = synchronized { id_count += 1; "j" + id_count }
-
-
-
   /** Isabelle environment **/
 
   /* platform prefixes */