Why would we do this? The configuration file we are reading back was
written by NetworkManager in the first place.

Maybe when assuming a connection after restart, this information could
be interesting. It however is not actually relevant.

Note how nm_dhcp_client_get_client_id() has only very few callers.

  - nm_device_spawn_iface_helper() in 'nm-device.c'. In this case,
    we either should use the client-id which we used when starting
    DHCP, or none at all.

  - ip4_start() in 'nm-dhcp-dhclient.c', but this is before starting
    DHCP client and before it was re-read from configuration file.

  - in "src/dhcp/nm-dhcp-systemd.c", but this has no effect for
    the dhclient plugin.