Remove references to global webhooks REST API preview

[jdennes]

Why:

I graduated this preview way back on 18 Sep 2019 in https://github.com/github/github/pull/124420 but the docs were not updated.

What's being changed:

I'm removing references to the "superpro" REST API preview. Is there a convenient way of updating all the masses of references in lib/rest/static/? I haven't updated that manually.

Check off the following:

• All of the tests are passing.
• I have reviewed my changes in staging. (look for the deploy-to-heroku link in your pull request, then click View deployment)
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[github-actions]

Thanks for contributing! We do not accept community changes to these files at this time.

• '.github/workflows/**'
• '.github/CODEOWNERS'
• 'translations/**'
• 'assets/fonts/**'
• 'data/graphql/**'
• 'lib/graphql/**'
• 'lib/redirects/**'
• 'lib/webhooks/**'