remove development only setting for new gradebook feature flag
remove development only setting for new gradebook feature flag

Releases the new gradebook from the confines of development and opens
it to site admins to turn it on for accounts.

closes CNVS-37250

test plan:
 - Confirm the feature flag shows up for site admins in an environment
   that runs rails in production

Change-Id: I43e0e009240ae58c2615cecc3428005d92181866
Reviewed-on: https://gerrit.instructure.com/114460
Reviewed-by: Jeremy Neander <20252aad5efdc666f083d82ab63ed0f9f72a043e@instructure.com>
Reviewed-by: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>
Tested-by: Jenkins
QA-Review: KC Naegle <9fbecd016338f9910821a8a5d5d0ae4fc9abfaa5@instructure.com>
Product-Review: Keith T. Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>

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