**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
count += (
                session.query(TI)
                .filter(
                    TI.dag_id == self.dag_id,
                    TI.execution_date == self.execution_date,
                    TI.task_id.in_(schedulable_ti_ids),
                )
                .update({TI.state: State.SCHEDULED}, synchronize_session=False)
            )
        
        if dummy_ti_ids:
            count += (
                session.query(TI)
                .filter(
                    TI.dag_id == self.dag_id,
                    TI.execution_date == self.execution_date,
                    TI.task_id.in_(dummy_ti_ids),
                )
                .update(
                    {
                        TI.state: State.SUCCESS,
                        TI.start_date: timezone.utcnow(),
                        TI.end_date: timezone.utcnow(),
                        TI.duration: 0,
                    },
                    synchronize_session=False,
                )
            )
        return count