DefaultTrigger#onMerge shouldn't clean up timers for merged windows.

The TriggerExecutor automatically cleans the trigger in all windows
that no longer exist.

----Release Notes----

[]
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=90673162

sdk/src/main/java/com/google/cloud/dataflow/sdk/transforms/windowing/DefaultTrigger.java

@@ -48,10 +48,6 @@ public TriggerResult onElement(TriggerContext<W> c, OnElementEvent<W> e) throws
 
   @Override
   public TriggerResult onMerge(TriggerContext<W> c, OnMergeEvent<W> e) throws Exception {
-    for (W oldWindow : e.oldWindows()) {
-      c.deleteTimer(oldWindow, TimeDomain.EVENT_TIME);
-    }
-
     c.setTimer(e.newWindow(), e.newWindow().maxTimestamp(), TimeDomain.EVENT_TIME);
     return TriggerResult.CONTINUE;
   }