timer: Forget about the block to help the GC

After a timer has been canceled we can forget about the block
that was supposed to be canceled. It is only a matter of time
until the timer will be discarded by the TimerScheduler.

Timer.st

@@ -58,6 +58,7 @@ Object subclass: Timer [
         <category: 'management'>
         "Remember that the timer is gone."
         schedule := nil.
+        block := nil.
     ]
 
     isCanceled [