[FC-0099] refactor: remove casbin redis watcher

[BryanttV]

Description

This PR removes the Casbin Redis watcher from the Open edX AuthZ engine. This change is made because we are now using a SyncEnforcer to automatically load the policy in this PR (#103).

Merge Checklist

Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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[bmtcril]

LGTM, thanks for taking care of this!

[mariajgrimaldi]

LGTM!