pickers: Remove `picker::ToMultiBuffer` action we did not land (#60979)

Missed this when shipping the pickers. The feature is still planned and
tracked in the tracking issue.

## 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 adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A
3e76813825Yara 🏳️‍⚧️ committed on 7/14/2026, 5:18:23 PM· committed by GitHubparentba0eea3
2 files changedLine totals unavailable