Fix schema export of overridden plays edge

[krishnangovindraj]

Usage and product changes

Make schema exports consistent with the TypeQL spec of overridden roles in 'plays' declarations being unscoped labels.

Implementation

• ThingTypeImpl.getSyntax() exports overridden role-types in 'plays' edges as unstopped labels.

• fixes vaticle/typeql#312

[vaticle-bot]

PR Review Checklist

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Trivial Change

• [x] This change is trivial and does not require a code or architecture review.

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