feat(block-theme): move coauthors RSS function to the Newspack Plugin

[laurelfulford]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This PR is a companion to Automattic/newspack-plugin#4629; they should be tested together.

Closes NPPD-1345

How to test the changes in this Pull Request:

1. Follow the steps in feat(block-theme): move co-authors RSS feed code to the Newspack Plugin newspack-plugin#4629.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[Copilot]

Pull request overview

This PR removes the theme-level implementation that injected Co-Authors Plus author lists into RSS/feeds, with the intent that this responsibility now lives in the Newspack Plugin.

Changes:

• Removed the newspack_coauthors_in_rss() filter callback.
• Removed the add_filter( 'the_author', ... ) hook that altered feed author output.

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