AO3-6636 Fixup speculation rules next-chapter prefetch

[domenic]

942f4a3 accidentally used the "eagerness" field, which is only supported in Chrome 121+. It isn't necessary in our case (the value we used, "eager", is already the default) so we can just remove it to fix prefetching in earlier versions of Chrome.

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? As discussed in the ticket, no automated tests are necessary for this. However, this time I tested manually using stable Chrome 119, and it works.
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
• Do you have fewer than 5 pull requests already open? If not, please wait
  until they are reviewed and merged before creating new pull requests.

Issue

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

Purpose

Fixes the speculation rules prefetch feature to work on earlier Chrome versions.

Testing Instructions

See the Jira ticket

References

• AO3-6636 Add speculation rules next-chapter prefetch #4661
• AO3-6636 Add speculation rules next-chapter prefetch (#4661) #4665

Credit

Domenic Denicola, he/him