**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._kill_process()
        if not self._process.is_alive():
            self._done = True
            self.log.debug("Waiting for %s", self._process)
            self._process.join()
            self._parent_channel.close()
            return True
        return False
    @property
    def result(self) -> Optional[Tuple[int, int]]:
        """
        :return: result of running DagFileProcessor.process_file()
        :rtype: tuple[int, int] or None
        """
        if not self.done:
            raise AirflowException("Tried to get the result before it's done!")
        return self._result
    @property
    def start_time(self) -> datetime.datetime:
        """
        :return: when this started to process the file
        :rtype: datetime
        """
        if self._start_time is None:
            raise AirflowException("Tried to get start time before it started!")
        return self._start_time
    @property
    def waitable_handle(self):
        return self._process.sentinel
class DagFileProcessor(LoggingMixin):