shairport-sync: don't daemonize

This flag causes the shairport-sync server to attempt to daemonize, but it looks like systemd is already handling that. With the `-d` argument, shairport-sync immediately exits—it seems that something (systemd I'm guessing?) is sending it SIGINT or SIGTERM.

The [upstream systemd unit](https://github.com/mikebrady/shairport-sync/blob/master/scripts/shairport-sync.service.in#L10) doesn't pass `-d`.