[css-contain] Fix bikeshed warning

Author: frivoal

css-contain-1/Overview.bs

@@ -29,7 +29,6 @@ spec: css-backgrounds-3;
 	type: dfn; url: https://drafts.csswg.org/css-backgrounds-3/#corner-clipping; text: corner clipping
 </pre>
 <pre class=link-defaults>
-spec:css21; type:dfn; text:stacking context
 spec:css-display-3; type:property; text:display
 spec:css-ui-3; type:property; text:text-overflow
 spec:css-grid-1; type:property; text:grid

css-contain-2/Overview.bs

@@ -21,11 +21,6 @@ WPT Path Prefix: css/css-contain/
 </pre>
 
 <pre class=link-defaults>
-spec:css21;
-	type:dfn; text:stacking context
-	type:property;
-		text:content
-		text:counter-increment
 spec:css-display-3; type:property; text:display
 spec:css-ui-3; type:property; text:text-overflow
 spec:css-ui-4; type:property; text:pointer-events