This reverts commit 317462be.

Reason for revert: Landed prematurely.

Original change's description:
> [compiler] Make BigInts bg-serialized
>
> BigInts are immutable after initialization, thus an acquire-release
> synchronization point is sufficient to read safely from the background
> thread. This CL introduces the `length` field as that sync point.
>
> Bug: v8:7790
> Change-Id: I977f30836b311c3851896dd29e708f78a090f547
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2854745
> Auto-Submit: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Michael Stanton <mvstanton@chromium.org>
> Commit-Queue: Michael Stanton <mvstanton@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#74245}

Tbr: mvstanton@chromium.org
Bug: v8:7790
Change-Id: I0fab1a8e161c75267aead3735708e26620686fa6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2854749
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74247}