[scroll-animations-1] Remove <custom-ident> from scroll() notation (w3c#7651)

Fixes w3c#7046.

Co-authored-by: Anders <andruud@dib.lan>

Author: andruud

scroll-animations-1/Overview.bs

@@ -136,7 +136,7 @@ spec: cssom-view-1; type: dfn;
 	<pre class="prod">
 		<dfn>scroll()</dfn> = scroll( <<axis>>? <<scroller>>? )
 		<dfn noexport><<axis>></dfn> = block | inline | vertical | horizontal
-		<dfn noexport><<scroller>></dfn> = root | nearest | <<custom-ident>>
+		<dfn noexport><<scroller>></dfn> = root | nearest
 	</pre>
 
 	By default,
@@ -173,12 +173,6 @@ spec: cssom-view-1; type: dfn;
 		<dt><dfn>root</dfn>
 		<dd>
 			Specifies to use the document viewport as the [=scroll container=].
-
-		<dt><dfn><<custom-ident>></dfn>
-		<dd>
-			Specifies to use the nearest ancestor [=scroll container=]
-			that has the specified 'container-name'. [[!CSS-CONTAIN-3]]
-			If none exist, the document viewport is used.
 	</dl>
 
 ### The {{ScrollTimeline}} Interface ### {#scrolltimeline-interface}