fix: remove unused notFoundString prop

[nolanlawson]

While playing with the storybook, I noticed a warning:

Warning: Failed prop type: The prop `notFoundString` is marked as required in `NotFound`, but its value is `undefined`.
    in NotFound (created by Category)
    in Category (created by NimblePicker)
    in div (created by NimblePicker)
    in div (created by NimblePicker)
    in NimblePicker (created by Picker)
    in Picker
    in WrapStory

This prop seems to be entirely unused, since this component actually uses i18n.notfound instead. I also can't find any documentation or any other references to it. It seems safe to remove.