AC_WPNav: remove unused variable following https://github.com/ArduPilot/ardupilot/pull/18076

libraries/AC_WPNav/AC_WPNav.h

@@ -261,7 +261,6 @@ protected:
     float       _wp_desired_speed_xy_cms;   // desired wp speed in cm/sec
     Vector3f    _origin;                // starting point of trip to next waypoint in cm from ekf origin
     Vector3f    _destination;           // target destination in cm from ekf origin
-    float       _track_error_xy;        // horizontal error of the actual position vs the desired position
     float       _track_scalar_dt;       // time compression multiplier to slow the progress along the track
     float       _terain_vel;            // maximum horizontal velocity used to ensure the aircraft can maintain height above terain
     float       _terain_accel;          // acceleration value used to change _terain_vel