The number of old-to-new slots was used as a heuristic in
GetMaxConcurrency() to control the number of background jobs. The
calculation already caused a bug that was fixed in
https://crrev.com/c/2593247 and isn't used in a major mark-compact.
Reduce complexity by removing that heuristic.

Change-Id: I88989974a94230b7d6f59846f5b0cce14b4118ca
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2649039Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72328}