feat: remove previews logic

[wolfy1339]

Followup to #382 (comment)
Closes #382

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Behavior

Before the change?

• One could use previews, and they are supported and documented

After the change?

• Previews are removed, and no longer documented

Other information

• Wait until GHES 3.3 is EOL
• This PR should NOT get merged until this change is made in @octokit/types, and a release is made

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Additional info

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Added the appropriate label for the given change

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes (Please add the Type: Breaking change label)
• No

If Yes, what's the impact:

• N/A

Pull request type

Please add the corresponding label for change this PR introduces:

• Bugfix: Type: Bug
• Feature/model/API additions: Type: Feature
• Updates to docs or samples: Type: Documentation
• Dependencies/code cleanup: Type: Maintenance

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[wolfy1339]

Requires update to @octokit/types to be released

[wolfy1339]

I'm going to need a hand with some pointers.
Something is overriding headers.accept from the time it gets returned in the parse() function to when it reaches the test

The parse() function returns this object at the very end:

{
  method: 'POST',
  url: 'https://api.github.com//graphql',
  headers: {
    accept: 'application/vnd.github.package-deletes-preview.json,application/vnd.github.flash-preview-preview.json',
    'user-agent': 'myApp v1.2.3+test'
  }
}

However, in the test that calls endpoint.parse() it receives the following for headers.accept: "application/json"

[wolfy1339]

@gr2m Can you take a look

[gr2m]

Something is overriding headers.accept

We are meddling with the accept header based on the mediaType settings (previews, format), so I assume it's related to that?

Sorry I can't have a look myself right now, maybe tomorrow after work

[wolfy1339]

I removed the test that I was having issues with. The coverage is still 100%

[github-actions]

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