**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
:param only_failed: Only clear failed tasks
        :type only_failed: bool
        :param only_running: Only clear running tasks.
        :type only_running: bool
        :param confirm_prompt: Ask for confirmation
        :type confirm_prompt: bool
        :param include_subdags: Clear tasks in subdags and clear external tasks
            indicated by ExternalTaskMarker
        :type include_subdags: bool
        :param include_parentdag: Clear tasks in the parent dag of the subdag.
        :type include_parentdag: bool
        :param dag_run_state: state to set DagRun to. If set to False, dagrun state will not
            be changed.
        :param dry_run: Find the tasks to clear but don't clear them.
        :type dry_run: bool
        :param session: The sqlalchemy session to use
        :type session: sqlalchemy.orm.session.Session
        :param dag_bag: The DagBag used to find the dags subdags (Optional)
        :type dag_bag: airflow.models.dagbag.DagBag
        :param exclude_task_ids: A set of ``task_id`` that should not be cleared
        :type exclude_task_ids: frozenset
        """
        if get_tis:
            warnings.warn(
                "Passing `get_tis` to dag.clear() is deprecated. Use `dry_run` parameter instead.",
                DeprecationWarning,
                stacklevel=2,
            )
            dry_run = True
        if recursion_depth: