问题描述：NacosMetadataReport#getServiceAppMapping 方法将监听器存入casListenerMap中，key是使用 DEFAULT_MAPPING_GROUP
作为group构建；但是 NacosMetadataReport#removeServiceAppMappingListener方法中，移除监听器时使用group不是 DEFAULT_MAPPING_GROUP，从而导致无法正常移除监听器

~~~
@Override
    public Set<String> getServiceAppMapping(String serviceKey, MappingListener listener, URL url) {
        String group = DEFAULT_MAPPING_GROUP;

        if (null == casListenerMap.get(buildListenerKey(serviceKey, group))) {
            addCasServiceMappingListener(serviceKey, group, listener);
        }
        String content = getConfig(serviceKey, group);
        return ServiceNameMapping.getAppNames(content);
    }

    @Override
    public void removeServiceAppMappingListener(String serviceKey, MappingListener listener) {
        MappingDataListener mappingDataListener = casListenerMap.get(buildListenerKey(serviceKey, group));
        if (null != mappingDataListener) {
            removeCasServiceMappingListener(serviceKey, group, listener);
        }
    }
~~~
public MappingDataListener(String dataId, String groupId) {
            this.serviceKey = dataId;
            this.dataId = dataId;
            this.groupId = groupId;
            this.listeners = new HashSet<>();
        }
        public void addListeners(MappingListener mappingListener) {
            listeners.add(mappingListener);
        }
        public void removeListeners(MappingListener mappingListener) {
            listeners.remove(mappingListener);
        }
        public boolean isEmpty() {
            return listeners.isEmpty();
        }
        @Override
        public void process(ConfigChangedEvent event) {
            if (ConfigChangeType.DELETED == event.getChangeType()) {
                return;
            }
            if (!dataId.equals(event.getKey()) || !groupId.equals(event.getGroup())) {
                return;
            }
            Set<String> apps = getAppNames(event.getContent());
            MappingChangedEvent mappingChangedEvent = new MappingChangedEvent(serviceKey, apps);
            listeners.forEach(listener -> listener.onEvent(mappingChangedEvent));
        }
    }
}