[ticket/9608] Remove use of references in captcha and other places

References are not really needed in PHP due to copy-on-write. Since
PHP5, objects are always passed around as identifiers, which means they
are mutable. So it is no longer required to pass these by reference
either.

PHPBB3-9608

phpBB/includes/captcha/plugins/phpbb_captcha_qa_plugin.php

@@ -99,9 +99,9 @@ class phpbb_captcha_qa
 	/**
 	*  API function
 	*/
-	function &get_instance()
+	function get_instance()
 	{
-		$instance =& new phpbb_captcha_qa();
+		$instance = new phpbb_captcha_qa();
 
 		return $instance;
 	}