[css-conditional-3] Remove duplicate interface w3c#3528 (w3c#4029)

CSSGroupingRule is defined by CSSOM.

Author: jugglinmike

css-conditional-3/Overview.bs

@@ -489,62 +489,6 @@ partial interface CSSRule {
 </pre>
 
 
-<h3 id='the-cssgroupingrule-interface'>
-The <code>CSSGroupingRule</code> interface</h3>
-
-The {{CSSGroupingRule}} interface represents an at-rule that contains other rules nested inside itself.
-
-<pre class='idl'>
-[Exposed=Window]
-interface CSSGroupingRule : CSSRule {
-    readonly attribute <a href="https://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSRuleList">CSSRuleList</a> cssRules;
-    unsigned long insertRule (CSSOMString rule, unsigned long index);
-    void deleteRule (unsigned long index);
-};
-</pre>
-
-<dl class='idl-attributes'>
-  <dt><code>cssRules</code> of type <code><a href="https://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSRuleList">CSSRuleList</a></code>, readonly
-  <dd>The <code>cssRules</code> attribute must return a <code>CSSRuleList</code>
-    object for the list of CSS rules nested inside the grouping rule.
-</dl>
-
-<dl class='idl-methods'>
-  <dt><code>insertRule(CSSOMString rule, unsigned long index)</code>, returns
-    <code>unsigned long</code>
-  <dd>
-    The <code>insertRule</code> operation must
-    insert a CSS rule <var>rule</var>
-    into the CSS rule list returned by <code>cssRules</code>,
-    such that the inserted rule will be at position <var>index</var>,
-    and any rules previously at <var>index</var> or higher
-    will increase their index by one.
-    It must throw INDEX_SIZE_ERR
-    if index is greater than <code>cssRules.length</code>.
-    It must throw SYNTAX_ERR
-    if <var>rule</var> has a syntax error and is unparseable;
-    this does not include syntax errors handled by error handling rules
-    for constructs inside of the rule,
-    but this does include cases where the string given
-    does not parse into a single CSS rule (such as when the string is empty)
-    or where there is anything other than whitespace or comments
-    after that single CSS rule.
-    It must throw HIERARCHY_REQUEST_ERR
-    if the rule cannot be inserted at the location specified,
-    for example, if an ''@import'' rule is inserted inside a group rule.
-
-    The return value is the <var>index</var> parameter.
-
-  <dt><code>deleteRule (unsigned long index)</code>, return <code>void</code>
-  <dd>
-    The <code>deleteRule</code> operation must
-    remove a CSS rule from
-    the CSS rule list returned by <code>cssRules</code> at <var>index</var>.
-    It must throw INDEX_SIZE_ERR
-    if index is greater than or equal to <code>cssRules.length</code>.
-</dl>
-
-
 <h3 id="the-cssconditionrule-interface">
 The <code>CSSConditionRule</code> interface</h3>