This reverts commit 11045926.

Reason for revert: Uncovered TSAN failure

Original change's description:
> [test] Avoid disabling of RCS during runtime
>
> Test was flaky since disabling of RCS at runtime is unsafe. Some code
> (e.g. TRACE_GC) is run only in case RCS is enabled and such code paths
> might also DCHECK that RCS is enabled.
> A background thread (sweeping in this case) could've already
> entered such a code path right before RCS is disabled. In this case the
> guard at the entry still saw that RCS is enabled but subsequent DCHECKs
> that ensure that RCS is enabled might fail.
>
> We could fix this by completing the sweeper tasks before disabling RCS
> but there might be other similar tasks which might be run at that point
> in the future (e.g. memory unmapper).
>
> Disabling the flag doesn't even seem to be needed for the initial
> regression test, so it seems simpler to just not disable RCS anymore.
>
> Bug: v8:12026
> Change-Id: I878f7dd9a7a4abb6a501f7a7651a1240ef2082a6
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3162043
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#76845}

Bug: v8:12026
Change-Id: I154817fc5ddeab68c69c745d08f3d77c48896dc4
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3162049
Auto-Submit: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#76848}