Error on every page due to invalid etc/events.xml file

Got error:

```
Invalid XML in file /var/www/magento/vendor/shipperhq/module-shipper/src/etc/events.xml:
Element 'config': Missing child element(s). Expected is ( event ).
Line: 8
```

As events.xml cannot contain empty config declaration

This was done in commit https://github.com/shipperhq/module-shipper/commit/76eda29ddda95741c5cda6c3fd4902d04a767131
