fix(ajax): remove unnecessary onAbort handling

Since the xhr object is closed over, nothing external will be able to abort it. At best this code will
be called and then swallowed because the Subscriber is already unsubscribed and will not allow the error to
propogate through the Subscriber chain. It is basically cruft and can be removed

src/observable/dom/ajax.ts

@@ -134,7 +134,6 @@ export class AjaxObservable<T> extends Observable<T> {
         xhr.onprogress = onProgress;
       }
       xhr.onerror = onError;
-      xhr.onabort = onAbort;
     }
 
     const contentType = headers['Content-Type'] ||
@@ -192,14 +191,6 @@ export class AjaxObservable<T> extends Observable<T> {
       subscriber.error(normalizeError(e, xhr, 'error'));
     }
 
-    function onAbort(e) {
-      done = true;
-      if (progressSubscriber) {
-        progressSubscriber.error(e);
-      }
-      subscriber.error(normalizeError(e, xhr, 'abort'));
-    }
-
     function onReadyStateChange(e) {
       if (xhr.readyState === 4) {
         processResponse(xhr, e);