Remove FLAG_IS_CMDLINE(UseSharedSpaces)

Since option UseSharedSpaces is obsolete, this code in arguments.cpp should be cleaned up to not use FLAG_IS_CMDLINE:

  if ((UseSharedSpaces && FLAG_IS_CMDLINE(UseSharedSpaces)) ||
      log_is_enabled(Info, cds)) {
    warning("Shared spaces are not supported in this VM");
    UseSharedSpaces = false;
    LogConfiguration::configure_stdout(LogLevel::Off, true, LOG_TAGS(cds));
  }

(noticed by [~ayang])