[rai] Fix extraction for recent raiplay.it updates (#27077)

[rai] Fix extraction for recent raiplay.it updates (#27077)

- Remove first test of RaiPlayIE: it is no longer available
- Make RaiPlayIE extension-agnostic (passing possible `.json' URLs is now
  supported too)
- Adjust RaiPlayLiveIE to recent raiplay.it updates.  Passing it as
  `url_transparent' is no longer supported (there is no longer an accessible
  ContentItem)
- Adjust RaiPlayPlaylistIE to recent raiplay.it updates and instruct it about
  ContentSet-s.
- Update a RaiIE test and remove two tests that are no longer availables

Thanks to @remitamine for the review!