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);
        }
    }
~~~
import static org.apache.dubbo.common.constants.CommonConstants.GROUP_KEY;
import static org.apache.dubbo.common.constants.RemotingConstants.BACKUP_KEY;
import static org.apache.dubbo.common.utils.StringConstantFieldValuePredicate.of;
import static org.apache.dubbo.common.utils.StringUtils.HYPHEN_CHAR;
import static org.apache.dubbo.metadata.MetadataConstants.REPORT_CONSUMER_URL_KEY;
import static org.apache.dubbo.metadata.ServiceNameMapping.DEFAULT_MAPPING_GROUP;
import static org.apache.dubbo.metadata.ServiceNameMapping.getAppNames;
/**
 * metadata report impl for nacos
 */
public class NacosMetadataReport extends AbstractMetadataReport {
    private NacosConfigServiceWrapper configService;
    /**
     * The group used to store metadata in Nacos
     */
    private String group;
    private Map<String, NacosConfigListener> watchListenerMap = new ConcurrentHashMap<>();
    private Map<String, MappingDataListener> casListenerMap = new ConcurrentHashMap<>();
    private MD5Utils md5Utils = new MD5Utils();
    public NacosMetadataReport(URL url) {
        super(url);
        this.configService = buildConfigService(url);
        group = url.getParameter(GROUP_KEY, DEFAULT_ROOT);
    }
    public NacosConfigServiceWrapper buildConfigService(URL url) {
        Properties nacosProperties = buildNacosProperties(url);
        try {
            configService = new NacosConfigServiceWrapper(NacosFactory.createConfigService(nacosProperties));
        } catch (NacosException e) {
            if (logger.isErrorEnabled()) {