This reverts commit 35a6eeec.

Reason for revert: TSAN failures like https://ci.chromium.org/ui/p/v8/builders/ci/V8%20Linux64%20TSAN/39084/overview

Original change's description:
> [off-thread] Allow off-thread top-level IIFE finalization
>
> Allow off-thread finalization for parallel compile tasks (i.e. for top-
> level IIFEs).
>
> This allows us to merge the code paths in BackgroundCompileTask, and
> re-enable the compiler dispatcher tests under the off-thread
> finalization flag. Indeed, we can simplify further and get rid of that
> flag entirely (it has been on-by-default for several releases now).
>
> Change-Id: I54f361997d651667fa813ec09790a6aab4d26774
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3226780
> Reviewed-by: Toon Verwaest <verwaest@chromium.org>
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#77615}

Change-Id: I6752470eebd594bad92c7cf4e58dbe5bac53598c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3255667Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Owners-Override: Shu-yu Guo <syg@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#77631}