How can I remove a unique constraint on a field when performing a down migration?

https://codeigniter.com/user_guide/dbmgm...rge-addkey

PHP Code:

up() {
        $this->forge->addKey('slug', false, true, 'slug_unique_name');
        $this->forge->processIndexes('pages');
} 


PHP Code:

down() {
        $this->forge->dropKey('pages', 'slug_unique_name');
}