**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
if self.state == State.RUNNING:
            return
        duration = self.end_date - self.start_date
        if self.state is State.SUCCESS:
            Stats.timing(f'dagrun.duration.success.{self.dag_id}', duration)
        elif self.state == State.FAILED:
            Stats.timing(f'dagrun.duration.failed.{self.dag_id}', duration)
    @provide_session
    def verify_integrity(self, session: Session = None):
        """
        Verifies the DagRun by checking for removed tasks or tasks that are not in the
        database yet. It will set state to removed or add the task if required.
        :param session: Sqlalchemy ORM Session
        :type session: Session
        """
        dag = self.get_dag()
        tis = self.get_task_instances(session=session)
        
        task_ids = set()
        for ti in tis:
            task_instance_mutation_hook(ti)
            task_ids.add(ti.task_id)
            task = None
            try:
                task = dag.get_task(ti.task_id)
            except AirflowException:
                if ti.state == State.REMOVED:
                    pass  
                elif self.state is not State.RUNNING and not dag.partial: