Remove PHP opening tags from wpseo-functions.php

[szepeviktor]

Context

Tame wild beasts 🐯

Summary

Popped up while generating wordpress-seo-stubs for static analysis.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label and noted the work hours.

Fixes php-stubs/wordpress-seo-stubs#5 (comment)

[szepeviktor]

Friendly ping 🏓