ntp: Remove version 4.2.8p8

We only need a client, so use systemd-timesyncd.

From systemd-timesyncd(8):

> The systemd-timesyncd service specifically implements only SNTP.  This
> minimalistic service will set the system clock for large offsets or
> slowly adjust it for smaller deltas. More complex use cases are not
> covered by systemd-timesyncd.

> timedatectl(1)´s set-ntp command may be used to enable and start, or
> disable and stop this service.

```
$ timedatectl
                      Local time: Mon 2018-03-05 13:09:59 UTC
                  Universal time: Mon 2018-03-05 13:09:59 UTC
                        RTC time: Mon 2018-03-05 13:09:59
                       Time zone: n/a (UTC, +0000)
       System clock synchronized: yes
systemd-timesyncd.service active: yes
                 RTC in local TZ: no
```