Update js/w3c/headers.js

When using ReSpec outside of W3C, the word "unofficial" in the heading is undesirable.
I plan to submit a feature request for a more generic template for use outside W3C, but meanwhile would be at least useful to remove the word "Unofficial" from the heading.
This shouldn't hurt any existing user while remove a limitation for others.

The "unofficial" CSS template also have have a "unofficial draft" logo, but that is easy to workaround in CSS in the document itself.