timedated: replace systemd-timedated-ntp.target logic with simpler scheme

The previous systemd-timedated-ntp.target was suffering by the problem
that NTP implementations enabled via the machanism could not be disabled
the obvious way on the "systemctl disable" command line. Replace
systemd-timedated-ntp.target by a list of implementations we try in
turn. The list is encoded in $pkgdatadir/ntp-units.