Remove the :closed pseudo-class (#11326)

This was resolved here:
#11039 (comment)

selectors-4/Overview.bs

@@ -2611,16 +2611,12 @@ Sound State: the '':muted'' and '':volume-locked'' pseudo-classes</h3>
 Element Display State Pseudo-classes</h2>
 
 <h3 id="open-state">
-Collapse State: the '':open'' and '':closed'' pseudo-class</h3>
+Collapse State: the '':open'' pseudo-class</h3>
 
 	The <dfn>:open</dfn> pseudo-class represents an element
 	that has both “open” and “closed” states,
 	and which is currently in the “open” state.
 
-	The <dfn>:closed</dfn> pseudo-class represents an element
-	that has both “open” and “closed” states,
-	and which is currently in the closed state.
-
 	Exactly what “open” and “closed” mean is host-language specific,
 	but exemplified by elements such as
 	HTML's <{details}>, <{select}>, and <{dialog}> elements,
@@ -2633,6 +2629,9 @@ Collapse State: the '':open'' and '':closed'' pseudo-class</h3>
 	or belongs to a ''display: none'' subtree)
 	can still be “open” and will match '':open''.
 
+	Note: A ":closed" pseudo-class might be added in the future
+	once the full set of things that support '':open'' is known.
+
 <h3 id="modal-state">
 Modal (Exclusive Interaction) State: the '':modal'' pseudo-class</h3>