{Isolate::per_isolate_thread_data_key()} is not even defined, and
{Isolate::isolate_key()} is unused.

R=mstarzinger@chromium.org

Bug: v8:8562
Change-Id: I490989510865903c702158e33621c9990052c2a8
Reviewed-on: https://chromium-review.googlesource.com/c/1425907Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58997}