This reverts commit 6596e8c6.

Reason for revert: Main bug was fixed.

Original change's description:
> [infra] Switch back to Xenial on some bots
>
> This tests the hypothesis that the current timeout problems are on
> Bionic bots only.
>
> Bug: v8:11818
> Change-Id: I68f84cda52ca392fbda5a400eb2bf136b7ee85a3
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2916816
> Auto-Submit: Michael Achenbach <machenbach@chromium.org>
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#74747}

Bug: v8:11818
Change-Id: Ib5f952dc6f23f3a98bb1d79ae5ce689e288d6727
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2940897
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74978}