All subtyping has been removed from the reference-types proposal. This
CL implements this proposal change now in V8.

R=manoskouk@chromium.org

Bug: v8:10556
Change-Id: I08ef064952278e03ea655461fa9f0c96426157c7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2222345
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68152}