Use spatial FAR if available, use temporal FAR if not

Currently we only consider the temporal FAR when making a decision about whether to publish a RAVEN coincidence event. Since there have been many improvements to making sure sky maps are available (see !689 (merged) !725 (merged) !755 (merged) !758 (merged)) we are now in a position to expect that this information should be available automatically. Therefore we can enable the functionality to use the coinc FAR with spatial information when considering whether to publish an event.

This merge request also removes a now unnecessary json.download step since ligo.raven.search.calc_signif_gracedb now returns the coincidence_far dictionary.