These are no longer enabled, so remove the code mitigation logic from
the codebase.

BUG=chromium:1003890

Change-Id: I536bb1732e8463281c21da446bbba8f47ede8ebe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3045704
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76256}