Use the existing move assignment operator instead.

R=ulan@chromium.org

Bug: v8:9183
Change-Id: Id7a4427da2bbf92d2954faba06e24afe64cb9818
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1594729Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61236}