Migrate Course `course-category` taxonomy to `topic`

[adamwoodnz]

Depends on #1009

Once all the Course course-category data has been moved into the common topic taxonomy we need to:

1. Update the Course archive page to use topic taxonomy
2. Unregister the course-category taxonomy and remove or replace any references in the UI

This seems to work to unregister it, however there were resulting errors:

add_action( 'admin_init', __NAMESPACE__ . '\unregister' );

/**
 * Unregister some taxonomies.
 */
function unregister() {
	// Unregister the Sensei course category in favour of the Topic taxonomy
	if ( taxonomy_exists( 'course-category' ) ) {
		unregister_taxonomy( 'course-category', 'course' );
	}
}