Remove incorrect xml mapping path

As the doctrine-model directory does not exist, the ``realpath`` method returns ``false``, causing this exception :

```
[Doctrine\Common\Persistence\Mapping\MappingException]                                                  
File mapping drivers must have a valid directory path, however the given path 0 seems to be incorrect!
```
Merge pull request #42 from EmmanuelVella/patch-1

Remove incorrect xml mapping path