Revert "[vm, gc] Allow for more parallelism in old-space GCs."

This reverts commit ae0740032ac912fb49be30789cff3f8fc8517c69.

Reason for revert: This is crashing on mac-release.

Crash logs: https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket.appspot.com/8880254409914739888/+/steps/test_results/0/logs/new_test_failures__logs_/0


Original change's description:
> [vm, gc] Allow for more parallelism in old-space GCs.
>
> Allow work stealing of new-space page when marking roots.
>
> Change-Id: I18fc22934bea26a37341216e88b13d1fb6f83b36
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/125767
> Reviewed-by: Martin Kustermann <kustermann@google.com>
> Reviewed-by: Siva Annamalai <asiva@google.com>
> Commit-Queue: Ryan Macnak <rmacnak@google.com>

TBR=kustermann@google.com,aam@google.com,rmacnak@google.com,asiva@google.com

Change-Id: I84c27cdd73a971f893ec968daaf5893c62c3a2e5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/148129
Reviewed-by: William Hesse <whesse@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>