Remove reference to undead tasks from documentation

[karenbraganz]

This PR removes references to undead tasks from the docs page here.

The term "undead task" is not used in the logs. Instead, if a running task has its state externally changed (what undead tasks is referring to in the docs), the logs display the below message where "x" is the new task state (source code).

"State of this instance has been externally set to <x>. Terminating instance."

There is no reference to undead tasks within this log message or anywhere else in Airflow (I think). Therefore, users do not search for the term "undead tasks" when they encounter this error. Including it in the documentation seems unhelpful and can even be confusing if users conflate it with zombie tasks.

Additionally, the line in the documentation above the code snippet states: "Below is the code snippet from the Airflow scheduler that runs periodically to detect zombie/undead tasks." This line is incorrect because the snippet only contains code for detecting zombies. The code to detect tasks whose state has been changed externally is in local_task_job_runner.py.

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[karenbraganz]

I have removed the code snippet.

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