nixos/enlightenment: remove some themes and xauth

- E already comes with a default icon theme
- There are already the gtk default Adwaita themes for gtk2, gtk3 and icons
- Remove gnome-icon-theme (from old gtk2)
- Remove tango-icon-theme
- Remove xauth (used by kdesu), as kdesu is not a componnent of E. If
  really needed it should be added in the system configuration.