Remove note from the "Handling document loss of full activity" section. (#354)

The reference to "active document" was broken after whatwg/html#6315 (it now
belongs to a "navigable").

Rather than trying to make sense of the changes to the HTML spec, just
remove the note since it was not adding a lot of information anyway.

Author: Raphael Kubo da Costa

index.html

@@ -662,11 +662,6 @@ <h3>
             </ol>
           </li>
         </ol>
-        <aside class="note">
-          Only documents presented to the user, thus with an associated
-          [=Document/browsing context=] can be <a>active documents</a>, so the
-          above steps are only relevant for these.
-        </aside>
       </section>
       <section>
         <h3>