liststore: Simplify code
liststore: Simplify code

This is also faster, though I doubt anyone's able to measure it.

The previous code was a more complicted way to do the same thing and it
was likely written the more complicated way because it fell out commit
758d7073a9485d770b25794f09a2dc32fef29ba1 when fixing
https://bugzilla.gnome.org/show_bug.cgi?id=795307

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib