[css-tables] Boxes don't have element ancestors

[Loirooriol]

CSS Tables says that fixup boxes

get their value from the closest element ancestor of the box.

But boxes don't have element ancestors. I think this text, added in #1059, was not necessary because it's already covered by CSS Display:

Unlike element-generated boxes, whose styles inherit strictly through the element tree, anonymous boxes (which only exist in the box tree) inherit through their box tree parentage.

[FremyCompany]

Sounds good, will fix.