[css-nav-1] Delete stale issue

Shadow DOM is not explicitely mentioend, but it is taken into account
via the various algorithms that the spec invokes, such as event
dispatching.

Author: frivoal

css-nav-1/Overview.bs

@@ -1009,8 +1009,6 @@ to fully define the behavior.
 Issue(19): The following currently does not account
 for <a href="https://github.com/WICG/overscroll-behavior">the proposed overscroll-behavior specification</a>.
 
-Issue(21): The following does not take shadow dom into account.
-
 
 <h3 id=grouping>
 Groupings of elements</h3>