networking.vpnc: remove option

This PR is part of the networking.* namespace cleanup.

The Cisco VPN module is currently of limited value since it just creates
config files but does not manage services. The same functionality can be
achieved by using _environment.etc_ instead.

It would be a different situation if we had a full service module. So if
you are annoyed by this change, please consider write a more featureful
module and put its options unter _services.networking.vpnc_.

Note that this change removes options for *Cisco VPN*, not
*networkmanager-vpn*.