This reverts commit 8c8e6b41.

Reason for revert: Makes Mac64 debug quite flaky: https://ci.chromium.org/p/v8/builders/ci/V8%20Mac64%20-%20debug/28766

Original change's description:
> [offthread] Serialize new-space objects into old space
> 
> If an object made it into serialization, it probably deserves to be in
> old space when it is deserialized. This will also make off-thread
> deserialization simpler, as there is no off-thread new space.
> 
> Bug: chromium:1075999
> Change-Id: Icabf2f0ae0a3e0205a1094dd0ffe675e69bd1d8e
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2184291
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#67606}

TBR=ulan@chromium.org,jgruber@chromium.org,leszeks@chromium.org

Change-Id: I0baca3c3b7985fcda6b9055f93111ec99c207d29
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:1075999
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2183925Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67615}