gz-bridge: use correct prev_timestamp for dt calc

- with the addition of the navsat plugin in PR#22638, the callback would
reassign the previous timestamp used in the calculations of the
angular_velocity causing derivative type noise in the groundtruth
measurements

src/modules/simulation/gz_bridge/GZBridge.cpp

@@ -719,8 +719,6 @@ void GZBridge::navSatCallback(const gz::msgs::NavSat &nav_sat)
 		updateClock(nav_sat.header().stamp().sec(), nav_sat.header().stamp().nsec());
 	}
 
-	_timestamp_prev = time_us;
-
 	// initialize gps position
 	if (!_pos_ref.isInitialized()) {
 		_pos_ref.initReference(nav_sat.latitude_deg(), nav_sat.longitude_deg(), hrt_absolute_time());