This reverts commit ad65e584.

Reason for revert: Regresses because of the same reasons for older OSX versions, see https://chromeperf.appspot.com/report?sid=4f570d131a97fdc45bc479f6e526d9caaa388eefca08242ef66cbf3b7eb850e4&start_rev=741028&end_rev=742546

Original change's description:
> heap: Remove OSX restriction for maximum parallelism
> 
> Change-Id: I582af41a8b10957b8fc7acb388871b1d51783a8c
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2062395
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#66317}

TBR=ulan@chromium.org,mlippautz@chromium.org

Change-Id: I1812a39f2f70fd2b00d77b3cfd49edcd4e967d6d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2064212Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66334}