phpbb_default_captcha::delete_code() takes no argument but uses $confirm_id
global $db, $user;
$sql = 'DELETE FROM ' . CONFIRM_TABLE . "
WHERE confirm_id = '" . $db->sql_escape($confirm_id) . "'
AND session_id = '" . $db->sql_escape($user->session_id) . "'
AND confirm_type = " . $this->type;
I'm not quite sure why that function is still present as it seems like it isn't used anymore:
// $this->delete_code(); commented out to allow posting.php to repeat the question
Anyhow, I think that $confirm_id is supposed to be $this->confirm_id.