Bug 1510351 - Remove unused code in PluginChild.jsm r=prathiksha

Differential Revision: https://phabricator.services.mozilla.com/D36524

--HG--
extra : moz-landing-system : lando

browser/actors/PluginChild.jsm

@@ -1229,10 +1229,5 @@ class PluginChild extends ActorChild {
       messageString,
       pluginID,
     });
-
-    // Remove the notification when the page is reloaded.
-    doc.defaultView.top.addEventListener("unload", event => {
-      this.hideNotificationBar("plugin-crashed");
-    });
   }
 }