Comment 2 for bug 1406140

This was an intentional change:
http://lists.freedesktop.org/archives/lightdm/2014-August/000661.html

From the NEWS file:

Overview of changes in lightdm 1.11.7

    * Use logind to provide the list of seats to use. Deprecate the old method
      of manually defining seats.
    * Add --show-config option that shows combined configuration.
    * Drop the surfaceflinger seat type. This was a temporary solution while
      Ubuntu Phone was migrating to Mir.