We were storing the pointer to the WasmModule both as a shared_ptr and
as a raw pointer. Maybe this had historical reasons, but now it's just
redundant.

R=thibaudm@chromium.org

Change-Id: Id72d102b6df804f93e3ab0235eeceef91a6dd8fb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2593334Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71782}