Plane: Allow wind relative dead reckoning when lift motors are assisting

When lift motors are assisting the wing during forward flight, the airspeed
and zero sideslip assumption are still sufficiently valid to keep navigating.
Without this patch a VTOL operating in FW mode without GPS and reliant on
air data for dead reckoning will stop navigating and lose its position estimate
if the lift motors start assisting.

ArduPlane/Plane.cpp

@@ -533,11 +533,6 @@ void Plane::update_fly_forward(void)
             ahrs.set_fly_forward(false);
             return;
         }
-
-        if (quadplane.in_assisted_flight()) {
-            ahrs.set_fly_forward(false);
-            return;
-        }
     }
 #endif