Remove allocation_mutex_ and use the already existing space_mutex
to protect the free list from concurrent allocation.

Bug: v8:10315
Change-Id: I8d823bcb80dfa49c6431a6db875488bc8d6ee8b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2352783Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69384}