**Apache Airflow version**: 2.1.0

**What happened**:

We use a python script that reads JSON configs from db to dynamically generate DAGs. Sometimes, when JSON Configs is updated, we expect DAGs to be removed in Airflow.  This doesn't always happen. From my observation, one of the following happens:
1) DAG is removed because the python script no longer generates it.
2) DAG still exists but run history is empty. When trigger the DAG, first task is stuck in `queued` status indefinitely.

**What you expected to happen**:
I expect the right behavior should be 
>1) DAG is removed because the python script no longer generates it.

**How to reproduce it**:
* Create a python script in DAG folder that dynamically generates multiple DAGs
* Execute these dynamically generated DAGs a few times
* Use Airflow Variable to toggle (reduce) the # of DAGs generated
* Examine if number of dynamically generated DAGs in the web UI


tests/dag_processing/test_processor.py
self, mock_send_email, mock_stats_incr, create_dummy_dag
    ):
        """
        Test that the dag file processor gracefully logs an exception if there is a problem
        sending an email
        """
        session = settings.Session()
        
        mock_send_email.side_effect = RuntimeError('Could not send an email')
        test_start_date = days_ago(2)
        dag, task = create_dummy_dag(
            dag_id='test_sla_miss',
            task_id='dummy',
            email='test@test.com',
            default_args={'start_date': test_start_date, 'sla': datetime.timedelta(hours=1)},
        )
        mock_stats_incr.reset_mock()
        session.merge(TaskInstance(task=task, execution_date=test_start_date, state='Success'))
        
        session.merge(SlaMiss(task_id='dummy', dag_id='test_sla_miss', execution_date=test_start_date))
        mock_log = mock.MagicMock()
        dag_file_processor = DagFileProcessor(dag_ids=[], log=mock_log)
        dag_file_processor.manage_slas(dag=dag, session=session)
        mock_log.exception.assert_called_once_with(
            'Could not send SLA Miss email notification for DAG %s', 'test_sla_miss'
        )
        mock_stats_incr.assert_called_once_with('sla_email_notification_failure')
    def test_dag_file_processor_sla_miss_deleted_task(self, create_dummy_dag):