- Swap() was not necessary as all uses merely required Merge()
- Remove unused empty Local ctor
- Use refrence for backref as it's always supposed to be non-null

Bug: v8:13193
Change-Id: Ide0a0de15185a67d028890371ae30528fd55a058
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3846863Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82648}