**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
"""This method is deprecated in favor of bulk_write_to_db"""
        warnings.warn(
            "This method is deprecated and will be removed in a future version. Please use bulk_write_to_db",
            DeprecationWarning,
            stacklevel=2,
        )
        return cls.bulk_write_to_db(dags, session)
    @classmethod
    @provide_session
    def bulk_write_to_db(cls, dags: Collection["DAG"], session=None):
        """
        Ensure the DagModel rows for the given dags are up-to-date in the dag table in the DB, including
        calculated fields.
        Note that this method can be called for both DAGs and SubDAGs. A SubDag is actually a SubDagOperator.
        :param dags: the DAG objects to save to the DB
        :type dags: List[airflow.models.dag.DAG]
        :return: None
        """
        if not dags: