Html template styles: remove span.underline.

Now we use `<u>` elements for underlined text, so this is
superfluous. (@mb21 correct me if I'm wrong.)

data/templates/styles.html

@@ -163,7 +163,6 @@ header {
 $endif$
 code{white-space: pre-wrap;}
 span.smallcaps{font-variant: small-caps;}
-span.underline{text-decoration: underline;}
 div.column{display: inline-block; vertical-align: top; width: 50%;}
 div.hanging-indent{margin-left: 1.5em; text-indent: -1.5em;}
 ul.task-list{list-style: none;}