**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, session: Session = None, execute_callbacks: bool = True
    ) -> Tuple[List[TI], Optional[callback_requests.DagCallbackRequest]]:
        """
        Determines the overall state of the DagRun based on the state
        of its TaskInstances.
        :param session: Sqlalchemy ORM Session
        :type session: Session
        :param execute_callbacks: Should dag callbacks (success/failure, SLA etc) be invoked
            directly (default: true) or recorded as a pending request in the ``callback`` property
        :type execute_callbacks: bool
        :return: Tuple containing tis that can be scheduled in the current loop & `callback` that
            needs to be executed
        """
        
        callback: Optional[callback_requests.DagCallbackRequest] = None
        start_dttm = timezone.utcnow()
        self.last_scheduling_decision = start_dttm
        with Stats.timer(f"dagrun.dependency-check.{self.dag_id}"):
            dag = self.get_dag()
            info = self.task_instance_scheduling_decisions(session)
            tis = info.tis
            schedulable_tis = info.schedulable_tis
            changed_tis = info.changed_tis
            finished_tasks = info.finished_tasks
            unfinished_tasks = info.unfinished_tasks
            none_depends_on_past = all(not t.task.depends_on_past for t in unfinished_tasks)
            none_task_concurrency = all(t.task.task_concurrency is None for t in unfinished_tasks)