Remove a useless button

This removes a print button that has been moved to purchase_order_webkit_with_notes module.
Tip:
It's better to keep the line break like that:
```
    'data': ['purchase_report.xml',
             ],
```

So the diff is only on 1 line (1 line removed) vs 3 lines here (2 removed, 1 added). Also it makes the diff more readable.