[ubuntu] Unity and Unity 2D missing from session selector in LightDM

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http://ubuntuforums.org – I recently installed the Cinnamon Desktop environment and after restarting noticed that the Unity and Unity 2D session options had disappeared from LightDM's session selector. I removed and purged all Cinnamon packages, but that didn't work. So then I reinstalled all unity-related packages to no avail either. I then removed, purged, and then reinstalled unity and unity-2d to no effect. How can this be fixed? (Hardware)