chore(ia): clean up connections

[miguelpeixe]

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:

Removes all references and code for the deprecated "Connections" wizard. The Fivetran connection has also been deprecated and completely removed (see p1714594679090649/1714590233.021889-slack-C06MPMZMF29)

How to test the changes in this Pull Request:

1. Checkout this branch, build the js, and confirm there are no errors
2. Navigate to "Newspack -> Settings"
3. With Newspack Manager configured, confirm you're able to connect and disconnect Google OAuth
4. Confirm you can change options in all sections of the "Connections" tab without issues
5. Navigate to all the other tabs and confirm no API errors pop up (no regression by removing the files)
6. Navigate to "Newspack -> Dashboard" and also confirm there are no API errors

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?