This operation would always fail in the UI because the git binary
already disallowed it. This PR just makes the UI more inline with what's
actually valid

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)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- git: Forbid main git worktree deletion from git worktree picker

---------

Co-authored-by: Eric Holk <eric@zed.dev>