[css-nav-1] Remove issue 3

Related issue(w3c#2324) looks like to be resolved.

Author: Jihye Hong

css-nav-1/Overview.bs

@@ -931,9 +931,6 @@ and aims for as much detail as necessary
 to fully define the behavior.
 </div>
 
-Issue(w3c/csswg-drafts#3376): The following currently does not account
-for <a href="https://github.com/WICG/overscroll-behavior">the proposed overscroll-behavior specification</a>.
-
 <h3 id=glossary>
 Glossary</h3>