Fix: Uninstall: OAuth Client ID

[n7studios]

Summary

uninstall.php incorrectly has the WooCommerce OAuth Client ID.

This PR resolves by using the WPForms OAuth Client ID. We can't use Plugin classes, methods or constants to instead use the INTEGRATE_CONVERTKIT_WPFORMS_OAUTH_CLIENT_ID constant, as they're not always reliably available during a Plugin's uninstallation.

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)