方法说明：
移除指定事件的某个监听器。
语法：
emitter.removeListener(event, listener)
接收参数：
event (string) 事件类型
listener (function) 已注册过的监听器
例子：
var callback = function(stream) {
console.log('someone connected!');
};
server.on('connection', callback);
// ...
server.removeListener('connection', callback);
源码：
// emits a 'removeListener' event iff the listener was removed
EventEmitter.prototype.removeListener = function(type, listener) {
var list, position, length, i;
if (!util.isFunction(listener))
throw TypeError('listener must be a function');
if (!this._events || !this._events[type])
return this;
list = this._events[type];
length = list.length;
position = -1;
if (list === listener ||
(util.isFunction(list.listener) && list.listener === listener)) {
delete this._events[type];
if (this._events.removeListener)
this.emit('removeListener', type, listener);
} else if (util.isObject(list)) {
for (i = length; i-- > 0;) {
if (list[i] === listener ||
(list[i].listener && list[i].listener === listener)) {
position = i;
break;
}
}
if (position < 0)
return this;
if (list.length === 1) {
list.length = 0;
delete this._events[type];
} else {
list.splice(position, 1);
}
if (this._events.removeListener)
this.emit('removeListener', type, listener);
}
return this;
};