Remove duplicate demo link in doc viewer

[Haprog]

Tested locally (after running polymer analyze vaadin-* > analysis.json) that this change gets rid of the extra "demo" link in the doc viewer. This will probably also fix it for webcomponents.org

Should fix #43 when there is a new version in webcomponents.org.

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[Haprog]

My testing showed that if there is no @demo annotation, there are no demo links. And if there is anything after if on the same line, the demo link is duplicated (and the link title and URL of one or both links change in very weird manner). If the line only has @demo then there is only one link (as wanted) and the link works correctly, but there doesn't seem to be any way in this case to modify the link text (it will be lowercase "demo") other than directly editing analysis.json after running polymer analyze vaadin-* > analysis.json.

I think this weird behaviour of duplicate demo link and how they behave when there's anything after the @demo on the line is a bug in polymer analyzer and due to this being a special case of "pseudo element" documentation instead of documenting a real custom element (since vaadin-icons.html does not define a custom element).

The same issue seems to affect the Polymer 2 version of iron-icon here: https://www.webcomponents.org/element/PolymerElements/iron-icons