If you don't need that, I would not bother with it and just use a regular gazebo_ros_control configuration. From your other question (#q306964) I have the impression that you're only really after kinematic visualisation / trajectory play-out. If that is the case then using Gazebo for this does not make much sense to me: it's a huge CPU hog and you're not using any of it then.
As I wrote in my comment: please don't consider moto_ros_interface.cpp as an example or how things should be done. It's a bit of a hack to get MotoROS-like behaviour out of a regular Gazebo+gazebo_ros_control setup. If you can clarify what you really want to do then perhaps we can see how that could be accomplished.