Revert "events: don't delete the listeners array in removeListener()"

This reverts commit 928ea564d1.

Keeping the original Array instance in-place essentially causes a memory leak
on EventEmitters that use an infinite number of event names (an incrementing
counter, for example), which isn't an unreasonable thing to want to do.

Fixes #3702.

lib/events.js

@@ -214,6 +214,8 @@ EventEmitter.prototype.removeListener = function(type, listener) {
 
     if (position < 0) return this;
     list.splice(position, 1);
+    if (list.length == 0)
+      delete this._events[type];
   } else if (list === listener ||
              (list.listener && list.listener === listener))
   {

test/simple/test-event-emitter-remove-listeners.js

@@ -42,15 +42,10 @@ function listener3() {
 }
 
 var e1 = new events.EventEmitter();
-var e1listeners = e1.listeners('hello');
 e1.on('hello', listener1);
-assert.equal(e1listeners.length, 1);
 e1.removeListener('hello', listener1);
 assert.deepEqual([], e1.listeners('hello'));
 
-// identity check, listeners array should be the same
-assert.equal(e1listeners, e1.listeners('hello'));
-
 var e2 = new events.EventEmitter();
 e2.on('hello', listener1);
 e2.removeListener('hello', listener2);
@@ -59,12 +54,8 @@ assert.deepEqual([listener1], e2.listeners('hello'));
 var e3 = new events.EventEmitter();
 e3.on('hello', listener1);
 e3.on('hello', listener2);
-var e3listeners = e3.listeners('hello');
-assert.equal(e3listeners.length, 2)
 e3.removeListener('hello', listener1);
-assert.equal(e3listeners.length, 1)
 assert.deepEqual([listener2], e3.listeners('hello'));
 
-assert.equal(e3listeners, e3.listeners('hello'));