Drop issues about the possibility of cancelable promises

According to #141 these are not going to eventuate after all.

This closes #141.

Overview.bs

@@ -790,10 +790,6 @@ The procedure to <dfn>reset an animation's pending tasks</dfn> for
 1.  If <var>animation</var> has a <a>pending pause task</a>, cancel that task.
 1.  <a lt="reject a Promise">Reject</a> <var>animation</var>'s <a>current ready
     promise</a> with a DOMException named "AbortError".
-
-    Issue: If cancelable promises materialize, we should probably cancel here
-    instead of rejecting.
-
 1.  Let <var>animation</var>'s <a>current ready promise</a> be the result of
     <a lt="create a new resolved Promise">creating a new resolved Promise
     object</a>.
@@ -1588,10 +1584,6 @@ as follows:
         <var>animation</var>.
     1.  <a lt="reject a Promise">Reject</a> the <a>current finished promise</a>
         with a DOMException named "AbortError".
-
-        Issue: If cancelable promises materialize, we should cancel here instead
-        of rejecting.
-
     1.  Let <a>current finished promise</a> be a new (pending) <a>Promise</a>
         object.
     1.  If <var>animation</var> is <em>not</em>
@@ -4874,14 +4866,8 @@ interface Animation : EventTarget {
 ::  The <a>play state</a> of this animation.
 :   <dfn attribute for=Animation>ready</dfn>
 ::  Returns the <a>current ready promise</a> for this object.
-
-    Issue: This should become a cancelable promise when (if) such things exist.
-
 :   <dfn attribute for=Animation>finished</dfn>
 ::  Returns the <a>current finished promise</a> for this object.
-
-    Issue: This should become a cancelable promise when (if) such things exist.
-
 :   <dfn attribute for=Animation>onfinish</dfn>
 ::  The event handler for the <a>finish event</a>.
 :   <dfn attribute for=Animation>oncancel</dfn>