It appears to be impossible to manually remove the track length if it previously has been set to something longer than 0:00. I was editing a 2 tracks long medium where the tracks were in the wrong order and track #1 had no length. I tried to switch the tracks around by manually removing the length from track #2 (and move it to track #1), but when I got to the edit preview tab both tracks had the same length. I never entered the edit so it might just be a display bug on the edit preview tab.
(It's possible to switch the tracks and their length by using the arrows, but I prefer not to use those because it creates messy looking edits.)