mc_pos_control: replace const with constexpr

src/modules/mc_pos_control/mc_pos_control_main.cpp

@@ -683,11 +683,9 @@ MulticopterPositionControl::run()
 				constraints.speed_up = _takeoff_speed;
 				// altitude above reference takeoff
 				const float alt_above_tko = -(_states.position(2) - _takeoff_reference_z);
-				// altitude threshold at which full control is enabled
-				const float alt_threshold = 0.5f; //at 0.5m above grouund, control all setpoints
 
 				// disable position-xy / yaw control when close to ground
-				if (alt_above_tko <= alt_threshold) {
+				if (alt_above_tko <= ALTITUDE_THRESHOLD) {
 					// don't control position in xy
 					setpoint.x = setpoint.y = NAN;
 					setpoint.vx = setpoint.vy = NAN;