**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
self.log.warning("Failed to get task '%s' for dag '%s'. Marking it as removed.", ti, dag)
                    Stats.incr(f"task_removed_from_dag.{dag.dag_id}", 1, 1)
                    ti.state = State.REMOVED
            should_restore_task = (task is not None) and ti.state == State.REMOVED
            if should_restore_task:
                self.log.info("Restoring task '%s' which was previously removed from DAG '%s'", ti, dag)
                Stats.incr(f"task_restored_to_dag.{dag.dag_id}", 1, 1)
                ti.state = State.NONE
            session.merge(ti)
        
        for task in dag.task_dict.values():
            if task.start_date > self.execution_date and not self.is_backfill:
                continue
            if task.task_id not in task_ids:
                Stats.incr(f"task_instance_created-{task.task_type}", 1, 1)
                ti = TI(task, self.execution_date)
                task_instance_mutation_hook(ti)
                session.add(ti)
        try:
            session.flush()
        except IntegrityError as err:
            self.log.info(str(err))
            self.log.info(
                'Hit IntegrityError while creating the TIs for ' f'{dag.dag_id} - {self.execution_date}.'
            )
            self.log.info('Doing session rollback.')
            
            session.rollback()
    @staticmethod
    def get_run(session: Session, dag_id: str, execution_date: datetime) -> Optional['DagRun']: