**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
self,
        execution_date: Optional[str] = None,
        run_id: Optional[str] = None,
        session: Optional[Session] = None,
    ):
        """
        Returns the dag run for a given execution date or run_id if it exists, otherwise
        none.
        :param execution_date: The execution date of the DagRun to find.
        :param run_id: The run_id of the DagRun to find.
        :param session:
        :return: The DagRun if found, otherwise None.
        """
        if not (execution_date or run_id):
            raise TypeError("You must provide either the execution_date or the run_id")
        query = session.query(DagRun)
        if execution_date:
            query = query.filter(DagRun.dag_id == self.dag_id, DagRun.execution_date == execution_date)
        if run_id:
            query = query.filter(DagRun.dag_id == self.dag_id, DagRun.run_id == run_id)
        return query.first()
    @provide_session
    def get_dagruns_between(self, start_date, end_date, session=None):