nixos: remove the obsolete 'services.openvpn.enable' option
nixos: remove the obsolete 'services.openvpn.enable' option

OpenVPN is enabled implicitly when configuring 'services.openvpn.servers', so the
"enable" option is meaningless since b2910df04e4ec3e707ad08074bd128dedf9b2c23.

Closes https://github.com/NixOS/nixpkgs/issues/9764.

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