[web-animations-1] Drop unnecessary and contradictory text about KeyframeAnimationOptions.timeline value

Fixes w3c#5701.

Author: birtles

web-animations-1/Overview.bs

@@ -5984,9 +5984,6 @@ animation.play();</pre>
 :   <dfn dict-member for=KeyframeAnimationOptions>timeline</dfn>
 ::  An optional value which, if present, specifies the <a>timeline</a>
     with which to associate the newly-created <a>animation</a>.
-    If missing, the <a>default document timeline</a> of the
-    {{Document}} <a lt="document associated with a window">associated</a>
-    with the {{Window}} that is the <a>current global object</a> is used.
 
 :   <dfn dict-member for=GetAnimationsOptions>subtree</dfn>
 ::  If true, indicates that [=animations=] associated with an