events: remove unneeded safety check code

In the process of creating nodejs#9865, it was discovered that the code
checking whether or not events was defined was unnecessary because
the only situation in which events would be undefined is if it is
monkeypatched by an external entity. This should be removed in order
to discourage this. If the test added in nodejs#9865 is merged, it will
need to removed on merge of this commit.

lib/events.js

@@ -423,17 +423,13 @@ EventEmitter.prototype.listeners = function listeners(type) {
   var ret;
   var events = this._events;
 
-  if (!events)
+  evlistener = events[type];
+  if (!evlistener)
     ret = [];
-  else {
-    evlistener = events[type];
-    if (!evlistener)
-      ret = [];
-    else if (typeof evlistener === 'function')
-      ret = [evlistener.listener || evlistener];
-    else
-      ret = unwrapListeners(evlistener);
-  }
+  else if (typeof evlistener === 'function')
+    ret = [evlistener.listener || evlistener];
+  else
+    ret = unwrapListeners(evlistener);
 
   return ret;
 };