AO3-6636 Remove rollout guard for speculation rules

[domenic]

See https://otwarchive.atlassian.net/browse/AO3-6636?focusedCommentId=360670.

Pull Request Checklist

• Have you read "How to write the perfect pull request"?
• Have you read the contributing guidelines?
• Have you added tests for any changed functionality? No tests needed, per discussion on the issue
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
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  until they are reviewed and merged before creating new pull requests.

Issue

https://otwarchive.atlassian.net/browse/AO3-6636

Purpose

Removes the rollout guard from the speculation rules next-chapter prefetch feature, since it interacts poorly with caching and was deemed unnecessary.

Testing Instructions

See instructions in https://otwarchive.atlassian.net/browse/AO3-6636.

References

The specific comment https://otwarchive.atlassian.net/browse/AO3-6636?focusedCommentId=360670.

Credit

Domenic Denicola, he/him