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);
        }
    }
~~~
return ServiceNameMapping.getAppNames(content);
    }
    private String getConfig(String dataId, String group) {
        try {
            return configService.getConfig(dataId, group);
        } catch (NacosException e) {
            logger.error(e.getMessage());
        }
        return null;
    }
    private void addCasServiceMappingListener(String serviceKey, String group, MappingListener listener) {
        MappingDataListener mappingDataListener = casListenerMap.computeIfAbsent(buildListenerKey(serviceKey, group), k -> new MappingDataListener(serviceKey, group));
        mappingDataListener.addListeners(listener);
        addListener(serviceKey, DEFAULT_MAPPING_GROUP, mappingDataListener);
    }
    private void removeCasServiceMappingListener(String serviceKey, String group, MappingListener listener) {
        MappingDataListener mappingDataListener = casListenerMap.get(buildListenerKey(serviceKey, group));
        if (mappingDataListener != null) {
            mappingDataListener.removeListeners(listener);
            if (mappingDataListener.isEmpty()) {
                removeListener(serviceKey, DEFAULT_MAPPING_GROUP, mappingDataListener);
                casListenerMap.remove(buildListenerKey(serviceKey, group), mappingDataListener);
            }
        }
    }
    public void addListener(String key, String group, ConfigurationListener listener) {
        String listenerKey = buildListenerKey(key, group);
        NacosConfigListener nacosConfigListener =
                watchListenerMap.computeIfAbsent(listenerKey, k -> createTargetListener(key, group));
        nacosConfigListener.addListener(listener);