**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 task: the task you want to add
        :type task: task
        """
        if not self.start_date and not task.start_date:
            raise AirflowException("Task is missing the start_date parameter")
        
        elif not task.start_date:
            task.start_date = self.start_date
        
        
        elif self.start_date:
            task.start_date = max(task.start_date, self.start_date)
        
        if not task.end_date:
            task.end_date = self.end_date
        
        
        elif task.end_date and self.end_date:
            task.end_date = min(task.end_date, self.end_date)
        if (
            task.task_id in self.task_dict and self.task_dict[task.task_id] is not task
        ) or task.task_id in self._task_group.used_group_ids:
            raise DuplicateTaskIdFound(f"Task id '{task.task_id}' has already been added to the DAG")
        else:
            self.task_dict[task.task_id] = task
            task.dag = self
            
            self._task_group.used_group_ids.add(task.task_id)
        self.task_count = len(self.task_dict)
    def add_tasks(self, tasks):