Move the wc_ship_to_billing_address_only() function to wc-order-functions.php

This needs to be present in the admin, so that the actions of sending order emails to work correctly.

```
Fatal error: Call to undefined function wc_ship_to_billing_address_only() in wp-content/plugins/woocommerce/templates/emails/plain/email-addresses.php on line 17
```