This reverts commit 34d053fbe8.
It was found that this patch (from 7 years ago) breaks upgrade path and
current behavior (user manual) of the sysmocom VPN.
It is expected that the systemd service openvpn.service is disabled by
default, and that the user simply has to enable/disable it to
enable/disable the sysmocom VPN.
This affects all images deployed until today, since until recently an
old sysmocom-openvpn-config package (from around 2013) was being used
during provisioning, instead of using the one we recently made available
into the regular 201705 feeds.