Get rid of deletion entries in the store buffer. Clearing a slot now
first empties the store buffer and then directly deletes the slot
from the remembered set.

Bug: v8:9454
Change-Id: I656db593a0478db3fa63324d7f3c6862b4b5e776
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1766130Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63359}