Cleanup BentoBarAndOverviewButtonExperiment from fieldtrial_testing_config.json.

R=minch@chromium.org

Bug: 1289680
Change-Id: I2a087db86387dbae6c20b65727bd84b12603e6f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3408255


Auto-Submit: Francois Pierre Doray <fdoray@chromium.org>
Reviewed-by: Min Chen <minch@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/main@{#962173}