Wrong variable used in parsing args

In https://github.com/wp-cli/wp-cli/blob/ae8be1523c0453ba43347d600c3f9caf8ff5bffc/php/WP_CLI/Configurator.php#L151, a variable `$positional_arg` is declared, but it's never used. Instead, the code uses `$positional`.

Correct me if I'm wrong, but I think that code only runs fine because `$positional` is always set if `$assoc_arg` is not null. Otherwise it would output `undeclared variable`.