[heap] Fix live bytes accounting in MinorMC

Live bytes were accounted twice. Once when object is marked black and
once explicitly by the marking task. Drop the accounting by the marking
task.

This should also help reduce binary size.

Bug: v8:12612, chromium:1331317
Change-Id: I43e90413309709662b6fcd0dfdec96cac1b5f231
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3732930


Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81429}