`zend_extension="/usr/lib/php5/20121212/xdebug.so"` removed for php.ini

- `zend_extension="/usr/lib/php5/20121212/xdebug.so"` removed in php.ini 

* this extension was added to the `conf.d` automatically
* I got `PHP Warning:  Module 'xdebug' already loaded in Unknown on line 0

```
> cat /etc/php5/apache2/conf.d/20-xdebug.ini
> zend_extension=/usr/lib/php5/20121212/xdebug.so
```