Revert "sum_tree: Replace rayon with futures (#41586) (#41846)

This causes the background executor to hang

Release Notes:

- N/A *or* Added/Fixed/Improved ...
5fc54986c7Lukas Wirth committed on 11/3/2025, 7:25:15 PM· committed by GitHubparentcb5055a
64 files changedLine totals unavailable