Some bad rebasing meant that we were still deleting on the main thread.

As an additional simplification, remove the specific deletion queue
mutex, and just use the compiler dispatcher mutex for the deletion queue
-- this avoids risks of deadlock when both are held.

Change-Id: Ifa4ead6ee3fd814d7f013dd14a5617456afc9f7f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3328785
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78335}