**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/dag_processing/processor.py
self,
        file_path: str,
        callback_requests: List[CallbackRequest],
        pickle_dags: bool = False,
        session: Session = None,
    ) -> Tuple[int, int]:
        """
        Process a Python file containing Airflow DAGs.
        This includes:
        1. Execute the file and look for DAG objects in the namespace.
        2. Execute any Callbacks if passed to this method.
        3. Serialize the DAGs and save it to DB (or update existing record in the DB).
        4. Pickle the DAG and save it to the DB (if necessary).
        5. Record any errors importing the file into ORM
        :param file_path: the path to the Python file that should be executed
        :type file_path: str
        :param callback_requests: failure callback to execute
        :type callback_requests: List[airflow.utils.dag_processing.CallbackRequest]
        :param pickle_dags: whether serialize the DAGs found in the file and
            save them to the db
        :type pickle_dags: bool
        :param session: Sqlalchemy ORM Session
        :type session: Session
        :return: number of dags found, count of import errors
        :rtype: Tuple[int, int]
        """