Now that write-protection of code memory is enabled everywhere and V8 is
fully W^X compliant, we can remove the permission mode in question.

R=hpayer@chromium.org
BUG=v8:6792

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I80fe95ac6bb0e2d1ad6d993154ce45d492d941be
Reviewed-on: https://chromium-review.googlesource.com/866855
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50770}