SITL: fixed rover altitude

thanks to Grant for noticing this

libraries/SITL/SIM_Rover.cpp

@@ -151,7 +151,6 @@ void SimRover::update(const struct sitl_input &input)
 
     // new position vector
     position += velocity_ef * delta_time;
-    position.z = -home.alt*0.01f;
 
     // update lat/lon/altitude
     update_position();