sum_tree: Remove Unit type (#35027)

This solves one ~TODO, as Unit type was a workaround for a lack of
ability to implement Summary for ().


Release Notes:

- N/A
1e2b0fcab6Piotr Osiewicz committed on 7/24/2025, 12:30:49 PM· committed by GitHubparent0af6900
2 files changedLine totals unavailable