This reverts commit 929b83fb.

Reason for revert: https://ci.chromium.org/ui/p/v8/builders/ci/V8%20Linux64%20UBSan/18725/overview

Original change's description:
> [heap] Attach to shared isolate after setting up main thread
>
> Attach to the shared isolate after the main thread was set up. Otherwise
> it could happen that a shared GC initiated from another isolate might
> see no threads are running and performs the safepoint operation in the
> middle of isolate deserialization.
>
> We use DisallowSafepoints to check that the isolate doesn't join a
> global safepoint before deserialization is complete. DisallowSafepoints
> used to prevent only invocations of Safepoint() but was updated to
> also prevent Park() and Unpark() invocations. Each state change could
> cause the thread to reach a safepoint, which would allow a shared GC
> to run.
>
> We now also DCHECK that every isolate has at least one local heap and
> that shared collections aren't started before deserialization is
> complete.
>
> Bug: v8:11708
> Change-Id: Iba3fb59dd951d5ee4fc9934158062287302fc279
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3221157
> Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Shu-yu Guo <syg@chromium.org>
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#77424}

Bug: v8:11708
Change-Id: I0633150b6b40b297a335a39bf1a087ca93592e04
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3225937Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77425}