Do not hide the warning implicitly

Even if COMPOSER_ALLOW_XDEBUG is set,
requires to also set COMPOSER_DISABLE_XDEBUG_WARN to hide the warning.

This undoes commit c5dcedd.

src/Composer/Console/Application.php

@@ -166,7 +166,7 @@ class Application extends BaseApplication
                 $io->writeError('<warning>Composer only officially supports PHP 5.3.2 and above, you will most likely encounter problems with your PHP '.PHP_VERSION.', upgrading is strongly recommended.</warning>');
             }
 
-            if (extension_loaded('xdebug') && (!getenv('COMPOSER_DISABLE_XDEBUG_WARN') || getenv('COMPOSER_ALLOW_XDEBUG'))) {
+            if (extension_loaded('xdebug') && !getenv('COMPOSER_DISABLE_XDEBUG_WARN')) {
                 $io->writeError('<warning>You are running composer with xdebug enabled. This has a major impact on runtime performance. See https://getcomposer.org/xdebug</warning>');
             }