dnsdist: Stop passing -u dnsdist -g dnsdist on systemd's ExecStart

dnsdist: Stop passing -u dnsdist -g dnsdist on systemd's ExecStart

This is useless as long as @service_user@ and @service_group@ are
set to 'dnsdist' (which is always true in our build system), and
would not have worked otherwise because dnsdist would have rejected
it with:
```
--gid/-g set on command-line, but dnsdist was started as a systemd service.
Use the 'Group' setting in the systemd unit file to set the group to run as
```