FaceGaze: Remove feature flag

FaceGaze is launched and is stable, so this change removes the flag for
the feature.

Bug: 427494093
Change-Id: Id342c5d8fa0930aa6c925d8c0c64dd1ffb059d83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6823986
Reviewed-by: Amanda Lin Dietz <aldietz@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1498209}
NOKEYCHECK=True
GitOrigin-RevId: db10adca2a36655b26c88a469af912ad936c1379