hide gradebook performance flag from site-admins
hide gradebook performance flag from site-admins

hide the gradebook performance flag from
site-admins on production.

closes CNVS-25397

test plan:
  - verify the feature flag is still visible
    for a site-admin in development and beta

Change-Id: I1981b3f9c6d64e16bb8a749bb1a6dabb0da6378d
Reviewed-on: https://gerrit.instructure.com/67946
Reviewed-by: Keith T. Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>
Tested-by: Jenkins
QA-Review: Amber Taniuchi <ef4bfd1e6d278f6bc7d4a478f02be2cb9581a0a5@instructure.com>
Product-Review: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>

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