**Apache Airflow version**: 2.0.0, LocalExecutor

**Environment**: Docker on Win10 with WSL, official Python3.8 image

**What happened**:

When a DAG is running, and I delete task from the running DAG, the scheduler fails. When using Docker, upon automatic restart of the scheduler, the scheduler just fails again, perpetually.

![image](https://user-images.githubusercontent.com/46958547/103558919-a89f6e80-4eb5-11eb-8521-19b04ee3c690.png)

Note: I don't _know_ if the task itself was running at the time, but I would guess it was.

**What you expected to happen**:

The scheduler should understand that the task is not part of the DAG anymore and not fail.

**How to reproduce it**:

- Create a DAG with multiple tasks
- Let it run
- While running, delete one of the tasks from the source code
- See the scheduler break
airflow/models/dagrun.py
max_number = cls.DEFAULT_DAGRUNS_TO_EXAMINE
        
        query = (
            session.query(cls)
            .filter(cls.state == State.RUNNING, cls.run_type != DagRunType.BACKFILL_JOB)
            .join(
                DagModel,
                DagModel.dag_id == cls.dag_id,
            )
            .filter(
                DagModel.is_paused.is_(False),
                DagModel.is_active.is_(True),
            )
            .order_by(
                nulls_first(cls.last_scheduling_decision, session=session),
                cls.execution_date,
            )
        )
        if not settings.ALLOW_FUTURE_EXEC_DATES:
            query = query.filter(DagRun.execution_date <= func.now())
        return with_row_locks(
            query.limit(max_number), of=cls, session=session, **skip_locked(session=session)
        )
    @staticmethod
    @provide_session
    def find(
        dag_id: Optional[Union[str, List[str]]] = None,
        run_id: Optional[str] = None,
        execution_date: Optional[datetime] = None,
        state: Optional[str] = None,