[rotorcraft] autopilot ground detect fix

the autopilot_ground_detected was reset before the nav would run and
trigger the ground detect exception, since the nav stuff runs on a lower frequency.
Autopilot_ground_detected should be reset in the nav function anyway.

part of #807

sw/airborne/firmwares/rotorcraft/autopilot.c

@@ -302,7 +302,7 @@ INFO("Using FAILSAFE_GROUND_DETECT: KILL")
 
   /* Reset ground detection _after_ running flight plan
    */
-  if (!autopilot_in_flight || autopilot_ground_detected) {
+  if (!autopilot_in_flight) {
     autopilot_ground_detected = FALSE;
     autopilot_detect_ground_once = FALSE;
   }