This:

- documents removal of Code's stub key field.
- removes SerializedCodeData's CodeStubKeys field.
- removes masm's custom self-reference marker mechanism.

Bug: v8:7777
Change-Id: Ie5c51bc895e508acdeb3994cf5558a2cf4c21540
Reviewed-on: https://chromium-review.googlesource.com/c/1367744
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58145}