The flag is causing build failures in some environments (when it is not
defined) and it is not actually needed since the behaviour of these FDs
across fork/exec isn't defined and this code is only used for testing.

Bug: chromium:1218005
Change-Id: If2bf6835705318079af56d920017accedaf769dd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3695583
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81032}