MapRDBGroupScan minor refactoring

Separate User workspace is not supported for MapR-DB due to specific format plugin implementation. It can be improved, but the change needs to be verified.

MapRDBGroupScan#getStorageConfig() can be the similar as for other format plugins:

@JsonProperty("storage")
public StoragePluginConfig getStorageConfig() {
  return formatPlugin.getStorageConfig()
}

See TODO in MapRDBGroupScan#getStorageConfig() method.
But it requires verifying: checking JSON Group Scan for MapR-DB

Then this improvement will allow to eliminate using TableFormatPlugin#getStoragePlugin() method fully.