Revert "Don't pull in lightdm or lightdm-gtk-greeter so SDDM becomes the default."

This reverts commit 520dd7e104 because the
change didn't actually work as intended, and should wait for a
lubuntu-default-settings rework to take place (because it will be excluded
either way).

desktop-qt

@@ -56,9 +56,6 @@ Task-Seeds: desktop-share core-qt
  * qtpass
  * qttranslations5-l10n             # Needed for proper translations support
 
- * !lightdm
- * !lightdm-gtk-greeter
-
 == Office ==
 
  * libreoffice-calc