Previously we needed to register the backing stores globally because
the embedder could create them from a raw pointer. This is no longer
possible after the removal of the old API.

The global backing store registry now keeps track only of wasm memory
backing stores.

Bug: v8:9380
Change-Id: Iffefbf14dcafc1f9ce0dc3613335c754c9cb649a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2763874Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73493}