Remove the task that is no longer being referenced.

Second phase for removing the task. The references to it were removed
earlier as a part of DEPR-41

cms/djangoapps/contentstore/tasks.py

@@ -550,15 +550,6 @@ def update_library_index(library_id, triggered_time_isoformat):
         LOGGER.debug(u'Search indexing successful for library %s', library_id)
 
 
-@task()
-def push_course_update_task(course_key_string, course_subscription_id, course_display_name):
-    """
-    Sends a push notification for a course update.
-    """
-    # TODO Delete once we've done a deploy where nothing is using this. DEPR-41
-    pass
-
-
 class CourseExportTask(UserTask):  # pylint: disable=abstract-method
     """
     Base class for course and library export tasks.