file-dialog-utils: remove ephy_file_dialog_add_shortcuts()

The underlying API was just removed in gtk!5396 (merged), so we have to stop using it. This is fine, though, because all it does is add the downloads folder to the file chooser sidebar, which is redundant because XDG directories are already there by default for a decade or so now. So this code is not useful and can disappear.

Thanks to Alexander for telling me exactly what to do here.