**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
callback = callback_requests.DagCallbackRequest(
                    full_filepath=dag.fileloc,
                    dag_id=self.dag_id,
                    execution_date=self.execution_date,
                    is_failure_callback=False,
                    msg='success',
                )
        
        elif unfinished_tasks and none_depends_on_past and none_task_concurrency and not are_runnable_tasks:
            self.log.error('Deadlock; marking run %s failed', self)
            self.set_state(State.FAILED)
            if execute_callbacks:
                dag.handle_callback(self, success=False, reason='all_tasks_deadlocked', session=session)
            elif dag.has_on_failure_callback:
                callback = callback_requests.DagCallbackRequest(
                    full_filepath=dag.fileloc,
                    dag_id=self.dag_id,
                    execution_date=self.execution_date,
                    is_failure_callback=True,
                    msg='all_tasks_deadlocked',
                )
        
        else:
            self.set_state(State.RUNNING)
        self._emit_true_scheduling_delay_stats_for_finished_state(finished_tasks)
        self._emit_duration_stats_for_finished_state()
        session.merge(self)
        return schedulable_tis, callback
    @provide_session
    def task_instance_scheduling_decisions(self, session: Session = None) -> TISchedulingDecision: