**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.log.warning(
                        "Task %s doesn't exist in DAG anymore, skipping SLA miss notification.", sla.task_id
                    )
                    continue
                tasks_missed_sla.append(task)
            emails: Set[str] = set()
            for task in tasks_missed_sla:
                if task.email:
                    if isinstance(task.email, str):
                        emails |= set(get_email_address_list(task.email))
                    elif isinstance(task.email, (list, tuple)):
                        emails |= set(task.email)
            if emails:
                try:
                    send_email(emails, f"[airflow] SLA miss on DAG={dag.dag_id}", email_content)
                    email_sent = True
                    notification_sent = True
                except Exception:
                    Stats.incr('sla_email_notification_failure')
                    self.log.exception("Could not send SLA Miss email notification for DAG %s", dag.dag_id)
            
            if notification_sent:
                for sla in slas:
                    sla.email_sent = email_sent
                    sla.notification_sent = True
                    session.merge(sla)
            session.commit()
    @staticmethod
    def update_import_errors(session: Session, dagbag: DagBag) -> None: