cups: remove xdg_utils dependency

`cups.desktop` that depends on some fixed version of `xdg-open` is not
particularly useful; it should use `xdg-open` from the environment
it's being run from.

As a side effect, one can now fiddle with `xdg_utils` package without
rebuilding pretty much every single one of graphical packages (they
all depend on `cups` through their graphical toolkits).