Delete needless env vars in new Heroku Review Apps
Delete needless env vars in new Heroku Review Apps

> The new version of review apps does not support parent apps -
> inherited configuration is specified in app.json or
> in the Pipeline’s configuration variables.

cf. Review Apps (New) - Heroku Dev Center
https://devcenter.heroku.com/articles/github-integration-review-apps#feature-changes

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