FlightTaskOrbit: only reapproach circle if center changed

The condition that the vehicle is more than 3m away from the circle line
was too sloppy. That often happens when the radius is changed by sticks.
A reapproach is only necessary when the center is moved and that's only
possible through the orbit command.

src/lib/FlightTasks/tasks/Orbit/FlightTaskOrbit.cpp

@@ -92,6 +92,9 @@ bool FlightTaskOrbit::applyCommandParameters(const vehicle_command_s &command)
 		}
 	}
 
+	// perpendicularly approach the orbit circle again when new parameters get commanded
+	_in_circle_approach = true;
+
 	return ret;
 }
 
@@ -179,12 +182,7 @@ bool FlightTaskOrbit::update()
 	setVelocity(v);
 
 	Vector2f center_to_position = Vector2f(_position) - _center;
-	const float distance_to_center = center_to_position.norm();
 
-	// perpendicularly approach the orbit circle if further away than 3 meters
-	if (!_in_circle_approach && !math::isInRange(distance_to_center, _r - 3.f, _r + 3.f)) {
-		_in_circle_approach = true;
-	}
 
 	if (_in_circle_approach) {
 		generate_circle_approach_setpoints();