This slot has become obsolete now that all CEntry stubs are builtins
(which are part of the rootset) and no longer need to be kept alive
explicitly by a slot in the frame.

R=verwaest@chromium.org
BUG=v8:8834

Change-Id: I7b791cc509ef800bcf7aa5faab31ddf35370f944
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1520725Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60267}