Modify _last_history function

The old method, using a many2one and _read_flat(), returned the wrong date if there were more than 1000 history lines.
this for loop is equivalent to writing

```python
result[reconcile_id] = last_history = self.pool['easy.reconcile.history'].search(
    cr, uid, [('easy_reconcile_id', '=', ids[-1])],
    limit=1, order='date desc'
)
```

I don't know if that's what you intended, but this change looks very odd to me...