The BigInt proposal got to stage 4 and integrated into the main spec.
Therefore the proposal tests are unnecessary and will be outdated soon.

R=thibaudm@chromium.org

Change-Id: I149de015f098a89333dd907bf5a4d18a36086c2b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2264095Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68542}