Previously in the git panel, double clicking a filename would trigger
both the first-click action *and* the second-click action.

This PR removes the second click handling since we don't have a good
pattern for single-and-double-click handling.

### Before



https://github.com/user-attachments/assets/f03e53ae-a330-4f60-812d-a45a2d3f5f30

### After



https://github.com/user-attachments/assets/e09493a1-6477-4d43-80ea-26322dd1f083




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

Release Notes:

- N/A