Remove a redundant default destructor definition and add final to
another which is overriding a virtual constructor.

Bug: v8:9396
Change-Id: I917f3c6058bfae8cbcd3c4ffb0817f78e8214552
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1807360
Commit-Queue: Dan Elphick <delphick@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63822}