Meta: stop exporting AbortSignal/signal abort

Helps with #1194.

Author: shaseley

dom.bs

@@ -1993,8 +1993,8 @@ object <var>signal</var>:
 </div>
 
 <div algorithm>
-<p>To <dfn export for=AbortSignal>signal abort</dfn>, given an {{AbortSignal}} object
-<var>signal</var> and an optional <var>reason</var>:
+<p>To <dfn for=AbortSignal>signal abort</dfn>, given an {{AbortSignal}} object <var>signal</var> and
+an optional <var>reason</var>:
 
 <ol>
  <li><p>If <var>signal</var> is [=AbortSignal/aborted=], then return.