code: def course_unregister_user(self, course, username=None):
          
          if username is None:
              username = self.session_username()
  
          # Needed if user belongs to a group
          self._database.aggregations.find_one_and_update(
              {"courseid": course.get_id(), "groups.students": username},
              {"$pull": {"groups.$.students": username, "students": username}})
  
          # If user doesn't belong to a group, will ensure correct deletion
          self._database.aggregations.find_one_and_update(
              {"courseid": course.get_id(), "students": username},
              {"$pull": {"students": username}})
  
          self._logger.info("User %s unregistered from course %s", username, course.get_id())
docs: Unregister a user to the course
          :param course: a Course object
          :param username: The username of the user that we want to unregister. If None, uses self.session_username()