less eventful shutdown: merely wait for scheduler to terminate;

--- a/src/Pure/Concurrent/future.ML	Tue Feb 26 12:46:47 2013 +0100
+++ b/src/Pure/Concurrent/future.ML	Tue Feb 26 12:50:52 2013 +0100
@@ -686,8 +686,7 @@
     SYNCHRONIZED "shutdown" (fn () =>
       while scheduler_active () do
        (Multithreading.tracing 1 (fn () => "SHUTDOWN: wait");
-        wait scheduler_event;
-        broadcast_work ()))
+        wait scheduler_event))
   else ();