ntp: make timesyncd the new default

- most nixos user only require time synchronisation,
  while ntpd implements a battery-included ntp server (1,215 LOCs of C-Code vs 64,302)
- timesyncd support ntp server per interface (if configured through dhcp for instance)
- timesyncd is already included in the systemd package, switching to it would
  save a little disk space (1,5M)