Replace check_if_subscriber_in_array() function

[n7studios]

Summary

Replaces the check_if_subscriber_in_array() function, as we query the API by email_address - so inspecting the response’s total_subscribers is sufficient.

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)