This reverts commit 6d342fa5.

Reason for revert: Needed to land:
https://crrev.com/c/3892788

Original change's description:
> [heap] Use std::unique_ptr for space_ array
>
> Document ownership with using std::unique_ptr<Space> for the space_
> array.
>
> Bug: v8:13267
> Change-Id: I12861d97cd52d2a8cf9ceb43a2f90008be87b2a3
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3890913
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#83187}

Bug: v8:13267
Change-Id: Ieeb29454e146ee763130c0031af3f7a48b4eec94
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3895811
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Owners-Override: Michael Achenbach <machenbach@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83192}