Remove 'math' extension for 'font-size: math'.

This is already in https://www.w3.org/TR/css-fonts-4/#font-size-prop
after w3c/csswg-drafts@09efeb0

Closes #62

index.html

@@ -4897,14 +4897,7 @@ <h3>The <code>math-shift</code> property</h3>
           </div>
       </section>
       <section id="the-math-script-level-property">
-        <h3>New value <code>math-depth</code> property and <code>font-size: math</code> value</h3>
-        <p>The <dfn><code>font-size</code></dfn> property
-          from <a data-cite="CSS-FONTS-4"></a>
-          is extended with a new value <code>math</code> value, indicating that
-          special mathematical scaling rules must be applied when determining
-          the computed value of the <code>font-size</code> property:
-        </p>
-        <code>&lt;font-size&gt; = <a data-cite="CSS-FONTS-4#font-size-prop">&lt;font-size-old&gt;</a> <a data-cite="CSS-VALUES-3#comb-one">|</a> math</code>
+        <h3>New value <code>math-depth</code> property</h3>
         <p>
           A new <a>math-depth</a> property is introduced to describe a notion
           of "depth" for each element of a mathematical formula, with respect to