PositionControl: temporarily remove direct thrust setpoint support

This commit temporarily breaks direct horizontal thrust setpoint execution
which is used by FlightTaskManualAltitude. This is necessary to allow for
PositionControl cleanup namely calculating the whole velocity PID in one
Vector3f formula.

Having this in a separate commit is useful since it reduces indentation
of a bigger code block.

src/lib/flight_tasks/tasks/ManualAltitude/FlightTaskManualAltitude.cpp

@@ -343,9 +343,7 @@ void FlightTaskManualAltitude::_updateSetpoints()
 		sp.normalize();
 	}
 
-	_thrust_setpoint(0) = sp(0);
-	_thrust_setpoint(1) = sp(1);
-	_thrust_setpoint(2) = NAN;
+	_thrust_setpoint.xy() = sp;
 
 	_updateAltitudeLock();
 	_respectGroundSlowdown();