Copter: use get_last_loop_time_s() for G_Dt

ArduCopter/ArduCopter.cpp

@@ -198,7 +198,7 @@ void Copter::perf_update(void)
 void Copter::loop()
 {
     scheduler.loop();
-    G_Dt = scheduler.get_filtered_loop_time();
+    G_Dt = scheduler.get_last_loop_time_s();
 }