fix: avoid redeclaration of global function

[dkoo]

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:

Avoids a possible redeclaration fatal if the thankyou.php custom template is loaded more than once. Related to Automattic/newspack-plugin#4251.

How to test the changes in this Pull Request:

1. While on feat(content-gate): implement restriction rules newspack-plugin#4251 and trunk of this repo, complete a modal checkout transaction on a page that renders content gates.
2. Observe a fatal error that prevents the success modal from loading: PHP Fatal error: Cannot redeclare Newspack_Blocks\newspack_blocks_replace_login_with_order_summary() (previously declared in /newspack-repos/newspack-blocks/src/modal-checkout/templates/thankyou.php:33) in /newspack-repos/newspack-blocks/src/modal-checkout/templates/thankyou.php on line 33
3. Check out this branch, repeat, confirm no error.

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?