gazebo_ros_wheel_slip: remove unused member data

gazebo_plugins/include/gazebo_plugins/gazebo_ros_wheel_slip.h

@@ -68,9 +68,6 @@ class GazeboRosWheelSlip : public WheelSlipPlugin
     /// \brief pointer to ros node
     private: ros::NodeHandle *rosnode_;
 
-    /// \brief Subscriber to detach control messages.
-    private: ros::Subscriber detachSub_;
-
     /// \brief Dynamic reconfigure server.
     private: dynamic_reconfigure::Server<gazebo_plugins::WheelSlipConfig>
                     *dyn_srv_;