Remove backtrace printing define from proxy so they won't appear in screenshot tests.

tests/PHPUnit/proxy/includes.php

@@ -9,10 +9,6 @@ if (!defined('PIWIK_USER_PATH')) {
     define('PIWIK_USER_PATH', PIWIK_INCLUDE_PATH);
 }
 
-if (!defined('PIWIK_PRINT_ERROR_BACKTRACE')) {
-    define('PIWIK_PRINT_ERROR_BACKTRACE', true);
-}
-
 require_once PIWIK_INCLUDE_PATH . '/core/Loader.php';
 require_once PIWIK_INCLUDE_PATH . '/core/EventDispatcher.php';
 require_once PIWIK_INCLUDE_PATH . '/core/Piwik.php';