cc:  @danilo-leal 

This setting is from when we had the git worktree picker in the agent
panel, now that it is in the menu bar it doesn't make sense to keep it.
We plan to add a similar feature in the future to handle the "new thread
== new git worktree" workflow

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #ISSUE

Release Notes:

- N/A