PIM-2219 Do not rely on a query builder to remove an attribute from a product

| Q                   | A
| ------------------- | ---
| Bug fix?            | yes
| New feature?        | no
| BC breaks?          | no
| Tests pass?         | yes
| Scenarios pass?     | yes
| Checkstyle issues?  | no
| Changelog updated?  | no
| Fixed tickets       |
| Doc PR              |

There is no such thing as a product value collection when using mongodb (values are embedded documents inside product).
Querying it the ProductValueRepository (in fact a DocumentRepository) causes serious error:

```
[2014-03-26 14:59:57] request.CRITICAL: Uncaught PHP Exception MongoException: "zero-length keys are not allowed, did you use $ with double quotes?" at /home/gildas/projects/pim/vendor/doctrine/mongodb/lib/Doctrine/MongoDB/Cursor.php line 483 {"exception":"[object] (MongoException: zero-length keys are not allowed, did you use $ with double quotes? at /home/gildas/projects/pim/vendor/doctrine/mongodb/lib/Doctrine/MongoDB/Cursor.php:483)"} []
```