**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
"""
        Set ``is_active=False`` on the DAGs for which the DAG files have been removed.
        Additionally change ``is_active=False`` to ``True`` if the DAG file exists.
        :param alive_dag_filelocs: file paths of alive DAGs
        :param session: ORM Session
        """
        log.debug("Deactivating DAGs (for which DAG files are deleted) from %s table ", cls.__tablename__)
        dag_models = session.query(cls).all()
        for dag_model in dag_models:
            if dag_model.fileloc is not None:
                if correct_maybe_zipped(dag_model.fileloc) not in alive_dag_filelocs:
                    dag_model.is_active = False
                else:
                    
                    
                    if not dag_model.is_active:
                        dag_model.is_active = True
            else:
                continue
    @classmethod
    def dags_needing_dagruns(cls, session: Session):