SingleSelect deprecates its own default prop. whoops.

Overview: <SingleSelect> has a deprecation warning on its tether prop. That's good! <SingleSelect> configures several default prop values, among them, tether. Whoops; that means the deprecation warning is always displayed, even when no tether prop is passed in.

<SingleSelect> should be refactored so the tether prop defaults are available when needed, but should not provide them in defaultProps.