device: don't assert the presence of old vpn configurations

device: don't assert the presence of old vpn configurations

When a VPN connection is active and the parent device becomes
unmanaged (due to device removal or system entering sleep), all
configurations are removed from the interface and the device is taken
down. After that, the VPN is disconnected and tries to remove the old
VPN configuration from device, causing the following assertion to
fail:

  _replace_vpn_config_in_list: assertion '!old' failed

Remove the assertion as @old can be missing for valid reasons.