[css3-ui] dropping nav-index (issue 25)

As part of the effort to stabilise css3-ui and move it along TR, we should drop nav-index from the level 3 spec, as it has issues, and no wide support.

This is listed as https://wiki.csswg.org/spec/css3-ui#issue-25

Note that this is only about nav-index, not about the directional nav-up/down/left/right properties which do have interoperable support.

 - Florian