Extensions are now always passed via unique_ptr and are owned by V8.
This CL removes the deprecated API where the embedder would own the
Extension, but has no mechanism for deleting it.

R=ulan@chromium.org

Bug: v8:8725
Change-Id: Icb83660fad9d04c66f8db2265091ebabcbb197c4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1514493Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60186}