This CL cleans up the existing experimental implementation of
conservative stack scanning. It retains the object start bitmap, to
evaluate it as a mechanism for resolving inner pointers, and the
conservative stack scanning visitor (which is currently not used).

The flag v8_enable_conservative_stack_scanning is kept and will be
used for experimental purposes. It currently does not imply any
other flag.

Bug: v8:10614
Bug: v8:12851

Change-Id: Id0ae0f437ed2601eed9ec634d2d1dd2f030d814e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3602516Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Jakob Linke <jgruber@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80365}