[jsfm] Fix the lint error in Document (#2368)

Remove the useless temporary variable `result` in the `fireEvent` method.

runtime/vdom/Document.js

@@ -183,14 +183,12 @@ export default class Document {
     if (domChanges) {
       updateElement(el, domChanges)
     }
-    let result
     let isBubble
     const $root = this.getRef('_root')
     if ($root && $root.attr) {
       isBubble = $root.attr['bubble'] === 'true'
     }
-    result = el.fireEvent(type, event, isBubble, options)
-    return result
+    return el.fireEvent(type, event, isBubble, options)
   }
 
   /**