Merge the two Flickr field types into one

The current existence of two Flickr Fields that both take photoset IDs might cause some confusion and seems unnecessary. When a combined field offers all formatting options that are currently covered by the two, we would have solved the issue.

The field 'Flickr photo set' got into the module at #409694: Flickr field for photo set with Flickr.com flash slideshow formatter. Probably it was easier to add a field that provided the requested features than building it into the existing field 'Flick Photo'.