Yoast SEO - Disabling the Primary category feature
As of Yoast SEO 3.1, we've added the ability to set a Primary category for a post. However, in some cases, you might not want to use this feature. That's why we've added a filter to allow you to completely disable it.
Please add the following code to your theme's
add_filter( 'wpseo_primary_term_taxonomies', '__return_empty_array' );