Fix for broken model.Model#remove by foreign key

Try something like:
```javascript
geddy.model.Post.remove({userId: this.id})
```
It will break with
```javascript
/usr/local/lib/node_modules/geddy/node_modules/model/lib/query/query.js:184
          datatype = descr.datatype;
                          ^
TypeError: Cannot read property 'datatype' of undefined
```

Tracking this down I've found that in geddy/lib/app/index.js there's a hook on 'beforeRemove' event that changes query conditions object adding 'model' and 'event' attributes in there.
The patch is just a simple workaround for this problem.
Not sure of the best way to fix.