[css-conditional] Make conditionText readonly.

As per #6819.

Author: emilio

css-conditional-3/Overview.bs

@@ -753,7 +753,7 @@ all the “conditional” at-rules,
 <pre class='idl' export>
 [Exposed=Window]
 interface CSSConditionRule : CSSGroupingRule {
-    attribute CSSOMString conditionText;
+    readonly attribute CSSOMString conditionText;
 };
 </pre>
 
@@ -781,25 +781,6 @@ js/conditional-CSSGroupingRule.html
     the result of serializing the associated condition.
 
     <wpt title=".conditionText returns serialization of condition."></wpt>
-
-    On setting the <code>conditionText</code> attribute these steps
-      must be run:
-
-    <ol>
-      <li>Trim the given value of white space.
-      <li>If the given value [=CSS/parses=]
-        as the appropriate condition grammar for the given rule
-        (such as <<supports-condition>> for ''@supports'', etc),
-        replace the associated CSS condition with the given value.
-      <li>Otherwise, do nothing.
-    </ol>
-
-    <!-- <wpt title="Setting .conditionText to a valid value changes the effective condition.">
-    js/CSSConditionRule.html
-    </wpt>
-    <wpt title="Setting .conditionText to an invalid value does nothing.">
-    js/CSSConditionRule.html
-    </wpt> -->
 </dl>