**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
"""
    DAG context is used to keep the current DAG when DAG is used as ContextManager.
    You can use DAG as context:
    .. code-block:: python
        with DAG(
            dag_id="example_dag",
            default_args=default_args,
            schedule_interval="0 0 * * *",
            dagrun_timeout=timedelta(minutes=60),
        ) as dag:
            ...
    If you do this the context stores the DAG and whenever new task is created, it will use
    such stored DAG as the parent DAG.
    """
    _context_managed_dag: Optional[DAG] = None
    _previous_context_managed_dags: List[DAG] = []
    @classmethod
    def push_context_managed_dag(cls, dag: DAG):