[css-scoping] Remove 'shadow host' dfn, since DOM now defines it.

Author: tabatkins

css-scoping/Overview.bs

@@ -323,7 +323,7 @@ Informative Explanation of Shadow DOM</h3>
 	at its root.
 	Children of the <a>shadow root</a> are ordinary elements and other nodes.
 	The element hosting the <a>shadow tree</a> is its <a>host</a>,
-	or <dfn export>shadow host</dfn>.
+	or <a>shadow host</a>.
 
 	The elements in a <a>shadow tree</a> are not <a>descendants</a> of the <a>shadow host</a> in general
 	(including for the purposes of Selectors like the <a>descendant combinator</a>).