Clean up some redundant variables after https://crrev.com/c/1449615.

R=mstarzinger@chromium.org

Bug: v8:8562
Change-Id: I44d46b037978f8feff7486cdc5c521c74c0a1c9e
Reviewed-on: https://chromium-review.googlesource.com/c/1449617
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59298}