Plane: avoid linking optflow on APM2

not used, just taking memory

ArduPlane/Plane.h

@@ -259,8 +259,10 @@ private:
     AP_Camera camera {&relay};
 #endif
 
+#if OPTFLOW == ENABLED
     // Optical flow sensor
     OpticalFlow optflow;
+#endif
 
     // Rally Ponints
     AP_Rally rally {ahrs};

ArduPlane/system.cpp

@@ -184,8 +184,10 @@ void Plane::init_ardupilot()
         }
     }
     
+#if OPTFLOW == ENABLED
     // make optflow available to libraries
     ahrs.set_optflow(&optflow);
+#endif
 
     // Register mavlink_delay_cb, which will run anytime you have
     // more than 5ms remaining in your call to hal.scheduler->delay