Remove support for legacy form checkout

[claudiulodro]

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 support for the legacy form-based WooCommerce checkout. Other than for backwards compatibility, I believe the main purpose it's still around is to support browsers that have JS disabled, but that's not something we support (especially since it wouldn't work with reCaptcha).

How to test the changes in this Pull Request:

Simplest way to verify that it's been removed:

1. Add a die( 'ATTEMPTING FORM CHECKOUT' ); here in your Woo plugin in the checkout_action method.
2. Put this script somewhere:

<?php

$site = 'http://newspackdev.local';

$body = [
	'woocommerce_checkout_place_order'    => 1,
];

var_dump( wp_remote_post( $site, [ 'body' => $body, 'timeout' => 50 ] ) );

4. Run it with wp eval-file scriptfile.php before applying this patch. Observe that the response contains that ATTEMPTING FORM CHECKOUT string within it.
5. Apply the patch. Run the script again. Observe that the response doesn't contain the string.
6. Verify regular Checkout screen and modal checkout work.

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?

[dkoo]

@claudiulodro this could be a hotfix, if we wanted it to go out sooner

[claudiulodro]

I'm OK sending it through the regular release flow just to benefit from the alpha release testing/process. It's pretty straightforward to add this code snippet to a site in an emergency in the meantime though.

[github-actions]

Hey @claudiulodro, good job getting this PR merged! 🎉

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Thank you! ❤️

[matticbot]

🎉 This PR is included in version 5.13.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀