services.flix: pin flix upstream to yellow retiolum ip4
jellyfin only listens on IPv4, but yellow.r also resolves to its
retiolum AAAA. nginx round-robined both addresses, so requests
hitting the v6 upstream failed and poisoned the upstream group,
causing intermittent 502 'no live upstreams' on flix.lassul.us.
Also drop the stale duplicate flix vhost from container-host.nix.