This reverts commit 84eec6e9.

Reason for revert: TSan issues (lock order inversion): https://ci.chromium.org/p/v8/builders/ci/V8%20Linux64%20TSAN%20-%20isolates/11658

Original change's description:
> [wasm] Use NumOutstandingCompilations() in BackgroundCompileJob:GetMaxConcurrency()
>
> This simplifies current_compile_job_ since ScheduleCompileJobForNewUnits
> is only called on the main thread.
>
> From pinpoint:
> v8:wasm:sync_instantiate:wall_time: 19.1% improvement
> v8-gc-incremental-step: 20.5% improvement
> https://pinpoint-dot-chromeperf.appspot.com/job/152920d8520000
>
> Change-Id: Id560080937f5439cf3321ce9306c7cae49e74798
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2442383
> Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
> Reviewed-by: Clemens Backes <clemensb@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#70386}

TBR=clemensb@chromium.org,etiennep@chromium.org

Change-Id: Iaa7df7fbfc56fcc7bf8400671f13210a1984885f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2456768Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70388}