**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,
        unfinished_tasks: List[TI],
        finished_tasks: List[TI],
        session: Session,
    ) -> bool:
        
        
        for ut in unfinished_tasks:
            if ut.are_dependencies_met(
                dep_context=DepContext(
                    flag_upstream_failed=True,
                    ignore_in_retry_period=True,
                    ignore_in_reschedule_period=True,
                    finished_tasks=finished_tasks,
                ),
                session=session,
            ):
                return True
        return False
    def _emit_true_scheduling_delay_stats_for_finished_state(self, finished_tis):
        """
        This is a helper method to emit the true scheduling delay stats, which is defined as
        the time when the first task in DAG starts minus the expected DAG run datetime.
        This method will be used in the update_state method when the state of the DagRun
        is updated to a completed status (either success or failure). The method will find the first
        started task within the DAG and calculate the expected DagRun start time (based on
        dag.execution_date & dag.schedule_interval), and minus these two values to get the delay.