Plane: use AP_RTC

Plane: AP_GPS now sets the system time directly

ArduPlane/ArduPlane.cpp

@@ -440,14 +440,6 @@ void Plane::update_GPS_10Hz(void)
 
                 next_WP_loc = prev_WP_loc = home;
 
-                // set system clock for log timestamps
-                uint64_t gps_timestamp = gps.time_epoch_usec();
-                
-                hal.util->set_system_clock(gps_timestamp);
-
-                // update signing timestamp
-                GCS_MAVLINK::update_signing_timestamp(gps_timestamp);
-
                 if (g.compass_enabled) {
                     // Set compass declination automatically
                     const Location &loc = gps.location();