Removes dead functions related to long removed SlotsBuffer class.

Bug: v8:10506
Change-Id: I79d6e77d2305e19048b62e2bc584fe986affd111
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2202908
Commit-Queue: Dan Elphick <delphick@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67833}