As set_description is never called and unsupported by the at-spi, we can
omit implementing it.
This means we can also omit get_description calls in various places, as
they'd just return the default value: NULL.
This code was supposed to work around a bad interaction between GOK and
Nautilus from 7 years ago.
If it still exists, the GOK developers may complain to the Nautilus
developers.
https://bugzilla.gnome.org/show_bug.cgi?id=137401
