Remove the async SNS topic.

def remove_async_sns_topic(self, lambda_name):
        """
        Remove the async SNS topic.
        """
        topic_name = get_topic_name(lambda_name)
        removed_arns = []
        for sub in self.sns_client.list_subscriptions()['Subscriptions']:
            if topic_name in sub['TopicArn']:
                self.sns_client.delete_topic(TopicArn=sub['TopicArn'])
                removed_arns.append(sub['TopicArn'])
        return removed_arns