FlightTasks: adapt tasks to member setpoints

The FlightTaskManual... tasks already had their internal
setpoint member variables. I switched them to use the
architecture with setpoint member variables as it was
implemented the commit before. They simplify a lot.

src/lib/FlightTasks/tasks/FlightTaskManual.cpp

@@ -98,12 +98,3 @@ bool FlightTaskManual::_evaluateSticks()
 		return false;
 	}
 }
-
-void FlightTaskManual::_resetToNAN()
-{
-	_thr_sp *= NAN;
-	_vel_sp *= NAN;
-	_pos_sp *= NAN;
-	_yaw_sp = NAN;
-	_yaw_rate_sp = NAN;
-}