timedated: always enable&start the service with highest priority

timedated: always enable&start the service with highest priority

This removes a special case that was implemented before: if some service
was already enabled, we'd treat it as having higher priority.

From https://bugzilla.redhat.com/show_bug.cgi?id=1735584#c4:
> Setting ntp off and on should give the same result as just setting it
> on. There should be no stickiness (hidden state). It should behave like
> running an ansible role.
>
> The other service might have been enabled because no other was installed at
> the time. If I install a new NTP service with a higher priority, setting ntp
> on should enable and start the new service, and disable all other. Also, if
> for some reason multiple services are enabled, after setting ntp on there
> should be only one enabled to avoid systemd selecting between them randomly
> on the next boot.