This function was only used for the write barrier since the store
buffer only stored slots and needed a way to get to the object's start.
Now that we insert into the remembered set directly from the write
barrier this isn't an issue anymore: the write barrier knows the
object start.

Change-Id: I701465ea40b7c4ee20404ecbcf3750e5fa6fd219
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1876049Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64518}