Memory masking was removed in https://crrev.com/c/3045704. The comment
and CHECK referred to the code to compute the memory mask, so it does
not apply any more. Thus remove it as well.

R=thibaudm@chromium.org

Bug: v8:12425
Change-Id: If11af4f2179dc01003ea6b532c6ac5696d6fa475
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3592953Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80043}