print backend: Fix list model handling in dispose

print backend: Fix list model handling in dispose

The print backends do some complicated dispose handling
where the implementations call gtk_print_backend_destroy().

Our tests (in particular, the templates test) trigger
situations where we use print backends after dispose,
and they can't handle the printers listmodel being
NULL at that time. So just remove the printers in
dispose, keep the empty liststore until finalize.