Ubuntu – Get rid of “unused” sessions in session switcher

kde4kde5lightdm

As shown in the screenshot below, whenever I enter the session switcher (it doesn't matter if it's via AltF2 (KRunner), the lock screen, etc.), I see an "Unused" entry for each VT that has an X session running in it.

KRunner showing "unused" sessions

This problem has persisted since Ubuntu 14.04 and followed me from LightDM to SDDM (and KDE4 to KDE5). How do I make the extra entries go away?