feat(ia): cleanup settings

[jaredrethman]

All Submissions:

• Have you followed the Newspack Contributing guideline?
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Changes proposed in this Pull Request:

• Created includes/wizards/newspack/class-syndication-section.php to house (IA) Newspack > Settings > Syndication php logic.
• Deleted settings and syndication files as they are no longer being used. Removed all references to Newspack\Settings methods.
• Migrated includes/wizards/class-settings.php 👉 includes/class-syndication.php. Migrated wizard related properties/methods to includes/wizards/newspack/class-syndication-section.php.

How to test the changes in this Pull Request:

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?