a few unused functions

Drive-By: Also clean up LoadSimd128 as LoadV128 and remove
Change-Id: I4cdee0fcb1e153309492026b4334af27afba7ec1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2584442
Commit-Queue: Junliang Yan <junyan@redhat.com>
Reviewed-by: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#71701}