Tracker: remove use of defunct state variables

AntennaTracker/GCS_Mavlink.cpp

@@ -433,8 +433,6 @@ void GCS_MAVLINK_Tracker::handleMessage(mavlink_message_t* msg)
         {
             // New home at wp index 0. Ask for it
             waypoint_receiving = true;
-            waypoint_request_i = 0;
-            waypoint_request_last = 0;
             send_message(MSG_NEXT_MISSION_REQUEST);
         }
         break;

AntennaTracker/GCS_Mavlink.h

@@ -44,4 +44,5 @@ private:
     MAV_MODE base_mode() const override;
     MAV_STATE system_status() const override;
 
+    bool waypoint_receiving;
 };