Also removes some obsolete gn definitions.

Bug: v8:8678
Change-Id: I418051177f059ab413b996a641ecf23f70d6c8bc
Reviewed-on: https://chromium-review.googlesource.com/c/1433789
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59163}