I'm talking about the red cross with tooltip "Remove disc". Go to http://musicbrainz.org/release/a2b21df9-0841-3095-b1dc-47ee20807cb2/edit and try clicking it, it does nothing. However, if you another disc (empty or otherwise), it's then possible to remove both disc 2 and disc 1. (What I really wanted to do was to replace the whole disc with another tracklist that I'd already edit to perfection, but removing and adding a new disc was the closest approximation I could find.)