This reverts commit c27b15b4.

Reason for revert: crbug.com/1314191

Original change's description:
> [heap] Enable --separate-gc-phases flag.
>
> Bug: v8:12503
> Change-Id: I81d02231b7e40fb0ed487de128ffaedcd3cd2126
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3571898
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Commit-Queue: Hannes Payer <hpayer@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#79817}

Bug: v8:12503, chromium:1314191
Change-Id: I82bd1db765344601cc0118da4a91345dd5001acf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3578655
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79936}