* debian/{preinst,postint,postrm}: cleanup, remove natty-specific code, and
  also gdm-specific code now that we use lightdm by default.
* usr/share/xubuntu/session.sh: dropped, now that lightdm sets the needed
  XDG vars, using startxfce4 is enough.
* usr/share/xsessions:
  - xubuntu.desktop.in, po/: dropped, not needed anymore.
  - xubuntu.desktop: added, generated from xfce.desktop.