Remove workaround for getNumSubscribers() when fixed

[stonier]

The api from image_common has a bug, but we do have a workaround for now:

  // Workaround https://github.com/ros-perception/image_common/issues/114
  // currNumSubscribers = ros_cam_pub_.getNumSubscribers();
  currNumSubscribers = std::max(
    this->count_subscribers(ros_cam_pub_.getTopic()),
    this->count_subscribers(ros_cam_pub_.getInfoTopic())
  );