v4 API: Use Traits from PHP SDK

[n7studios]

Summary

Uses the trait class from the PHP SDK for v4 API methods, removing the equivalent methods from the API class.

Further PRs to follow to reinstate tests.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

[noelherrick]

I didn't go over this with a fine-toothed comb since I assume that this was mostly moving code around. Let me know if there are places that need more attention.

[n7studios]

I didn't go over this with a fine-toothed comb since I assume that this was mostly moving code around. Let me know if there are places that need more attention.

That's fine, as it's using the PHP SDK's traits class. Further PR's to follow here with tests to make sure the API methods work through the WordPress ConvertKit_API class, and then we'll have some PR's on the main ConvertKit Plugin that implement this v4 API to further confirm no breaking changes. They'll be a lot smaller and easier to review.