Drive-by fix for x64 to remove unused temporary register.

Bug: v8:8460
Change-Id: Ib4e83bad4e14670ccd93dd10e16c5483eb0897b8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2283233
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68724}