Revert "terminal: Actually close process groups when the terminal is closed (#61467)" (#62399)

This reverts commit 6297c88f428a99741a7bfb33f31dfe98123bb8e4.

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fixes #62286
fixes #62095

https://github.com/zed-industries/zed/pull/61467 fixed its intended bug,
but at the same time introduced an issue where running tasks that would
cause new tasks to be terminated immediately.
https://github.com/zed-industries/zed/pull/62322 tried to fix that
forward, but was unsuccessful. In the mean-time I am going to revert the
original PR.

We can try to re-land the original bugfix in a future PR.

Release Notes:

- N/A