**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
"""Returns the latest date for which at least one dag run exists"""
        return session.query(func.max(DagRun.execution_date)).filter(DagRun.dag_id == self.dag_id).scalar()
    @property
    def latest_execution_date(self):
        """This attribute is deprecated. Please use `airflow.models.DAG.get_latest_execution_date` method."""
        warnings.warn(
            "This attribute is deprecated. Please use `airflow.models.DAG.get_latest_execution_date` method.",
            DeprecationWarning,
            stacklevel=2,
        )
        return self.get_latest_execution_date()
    @property
    def subdags(self):
        """Returns a list of the subdag objects associated to this DAG"""
        
        from airflow.operators.subdag import SubDagOperator
        subdag_lst = []
        for task in self.tasks:
            if (
                isinstance(task, SubDagOperator)
                or
                
                type(task).__name__ == 'SubDagOperator'
                or task.task_type == 'SubDagOperator'
            ):
                subdag_lst.append(task.subdag)
                subdag_lst += task.subdag.subdags
        return subdag_lst
    def resolve_template_files(self):