We can now tighten the return type of FindRootMap and remove some
related code.

Bug: v8:7790
Change-Id: I08325e7e4f4c9261c45770f7674b6644cc5c2b80
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3123411Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76564}