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

crates/picker/src/picker.rs

@@ -76,8 +76,6 @@ actions!(
         SetPreviewHidden,
         /// Opens the footer's actions menu.
         ToggleActionsMenu,
-        /// Take the picker's content and open it in a multibuffer
-        ToMultiBuffer,
         /// Toggles multi-select mode, in which clicking items adds them to
         /// the selection instead of opening them
         ToggleMultiSelect,