Allow AllocationObserver::Step to remove allocation observers as well.
They could already add new observers (to start incremental marking),
removing will be used when starting StressConcurrentAllocator from
an allocation observer.

Bug: v8:10315
Change-Id: I823ded2f9a408b3fa5269ee8416060d0cabb3162
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2357690Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69420}