**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


airflow/models/dag.py
"""
        Deactivate any DAGs that were last touched by the scheduler before
        the expiration date. These DAGs were likely deleted.
        :param expiration_date: set inactive DAGs that were touched before this
            time
        :type expiration_date: datetime
        :return: None
        """
        for dag in (
            session.query(DagModel)
            .filter(DagModel.last_parsed_time < expiration_date, DagModel.is_active)
            .all()
        ):
            log.info(
                "Deactivating DAG ID %s since it was last touched by the scheduler at %s",
                dag.dag_id,
                dag.last_parsed_time.isoformat(),
            )
            dag.is_active = False
            session.merge(dag)
            session.commit()
    @staticmethod
    @provide_session
    def get_num_task_instances(dag_id, task_ids=None, states=None, session=None):