[css-values-3] Remove the "string" <dfn>.

Author: tabatkins

css-values-3/Overview.bs

@@ -32,6 +32,7 @@ spec: css2; type: property; text: border-collapse
 spec: css-color-4; type: value; text: currentcolor
 spec: css-color-4; type: value; text: color
 spec: css-cascade-4; type: at-rule; text: @import
+spec: infra; type: dfn; text: string
 </pre>
 <style>
 code, small { white-space: nowrap }
@@ -473,7 +474,7 @@ Author-defined Identifiers: the <<custom-ident>> type</h3>
 <h3 id="strings">
 Quoted Strings: the <<string>> type</h3>
 
-	<dfn export lt="string">Strings</dfn> are denoted by <dfn id="string-value">&lt;string></dfn>
+	[=Strings=] are denoted by <dfn id="string-value">&lt;string></dfn>
 	and consist of a sequence of characters delimited by double quotes or
 	single quotes. They correspond to the <<string-token>> production
 	in the <a href="https://www.w3.org/TR/css-syntax/">CSS Syntax Module</a> [[!CSS3SYN]].