revoke task when clearing scheduled tasks;

remove unused code

courses/signals.py

@@ -163,7 +163,6 @@ def create_or_update_events(instance, type):
 
     else:
         # case when not published but there is no release date and there is prior event
-        # import ipdb;ipdb.set_trace()
         if type == 'course':
             # less DRY please
             try:
@@ -196,17 +195,18 @@ def create_or_update_events(instance, type):
 
         if event:
             event.delete()
-    # case when not published but there is not release date and there is prior task scheduled
-    if not instance.release_date and st.release_date and st.task_hash:
-        app.control.revoke(st.task_hash)
-        st.delete()
 
 
 def clear_scheduled_tasks(instance):
-    # import ipdb;ipdb.set_trace()
 
     content_type = ContentType.objects.get_for_model(instance)
-    ScheduledTask.objects.filter(content_type=content_type, object_id=instance.id).delete()
+    try:
+        st = ScheduledTask.objects.get(content_type=content_type, object_id=instance.id)
+        if st.task_hash:
+            app.control.revoke(st.task_hash)
+        st.delete()
+    except ScheduledTask.DoesNotExist:
+        pass
 
 
 @receiver(post_save, sender=Lesson)