[cssom] Reflect reality: CSSRule#cssText setter does nothing

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=29433.

Author: zcorpan

cssom/Overview.bs

@@ -1848,16 +1848,7 @@ obsolete by this specification. This value will not be re-allocated in the futur
 
 The <dfn attribute for=CSSRule>cssText</dfn> attribute must return a <a lt="serialize a CSS rule">serialization</a> of the
 <a for=/>CSS rule</a>.
-On setting the {{CSSRule/cssText}} attribute these steps must be run:
-<ol>
- <li>Let <var>new rule</var> be the returned value of <a lt="parse a CSS rule">parsing</a> the value.
- <li>If <var>new rule</var> is a syntax error, terminate this algorithm.
- <li>If the <a for=CSSRule>type</a> of <var>new rule</var> does not match the <a for=CSSRule>type</a> of
- the current object
- <a>throw</a> an
- {{InvalidModificationError}} exception.
- <li>Replace the current object with <var>new rule</var>.
-</ol>
+On setting the {{CSSRule/cssText}} attribute must do nothing.
 
 The <dfn attribute for=CSSRule>parentRule</dfn> attribute must return the <a for=CSSRule>parent CSS
 rule</a>.