remove beta tag from "Prefer HTML5 for video playback" feature flag

fixes #CNVS-17490

test-plan:
* go to the feature options tab on an accounts settings
* verify the beta tag is gone from "Prefer HTML5 for video playback"

Change-Id: If1cdc695df94513271751ced4edbc3cf55a47c6b
Reviewed-on: https://gerrit.instructure.com/45734
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>