[CSS21] remove mentions of percentage intrinsic widths/heights

In sections 10.3.2 and 10.6.2 CSS 2.1 has references to percentage
intrinsic widths/heights.

In discussions with the SVG working group just now, the SVG working
group believes that width/height on the root SVG element never
actually lead to a percentage intrinsic dimension.

Since these cases of percentage intrinsic dimensions actually all
lead to horrible results, we should probably remove mention of
percentage intrinsic dimensions from the specification, since we
don't actually have any use cases for them (and fix the relevant
tests in the test suite).

-David

-- 
L. David Baron                                 http://dbaron.org/
Mozilla Corporation                       http://www.mozilla.com/