Remove sentences about document with no focusable area

The PR whatwg#3647 removed "control group" concept, and most of them
were converted to Document. The "control group" could be empty
when e.g. no focusable element in a dialog element, but for a
document without any focusable element, its viewport is still
focusable.

Remove sentences talking about such a non-existent Document
condition.

source

@@ -72538,8 +72538,7 @@ END:VCARD</pre>
 
   <p>One <span>focusable area</span> in each <code>Document</code> is designated the <dfn>focused
   area of the document</dfn>. Which control is so designated changes over time, based on algorithms
-  in this specification. If a <code>Document</code> has no <span>focusable area</span>, it has no
-  <span data-x="focused area of the document">focused area</span>.</p>
+  in this specification.</p>
   <!-- TODO(kochi): update "in a document tree" for shadow trees -->
 
   <p><span data-x="focusable area">Focusable areas</span> in a <code>Document</code>
@@ -72962,9 +72961,6 @@ END:VCARD</pre>
 
   <hr>
 
-  <p>When a <span>focusable area</span> is added to an empty <code>Document</code>, it must be
-  designated the <span>focused area of the document</span>.</p>
-
   <p><dfn>Focus fixup rule</dfn>: When the designated <span data-x="focused area of the
   document">focused area of the document</span> is removed from that <code>Document</code> in some
   way (e.g. it stops being a <span>focusable area</span>, it is removed from the DOM, it becomes