Remove the GUI screenshots from Orca's documentation

* New screenshots are needed each time Orca's GUI changes
* New screenshots are needed each time GNOME's Gtk+ theme changes
* They are a pain for the i18n team to make (and often not being made)
* Most other GNOME documentation seems to lack them
* They don't really add much value in general, plus:
* Most of the primary consumers of Orca's documentation happen to be blind