Verify that package dhparam is required

Package dhparam might not be required. The only dependency is foris-controller-openvpn-plugin but core package for openvpn turris-cagen does not depend on it. turris-cagen also seems to generate dhparam itself. It seems that turris-cagen just does that for itself and it might not be required at all in the end.

Please @jschlehofer investigate.