delivery_roulier: fix call to dead code

this function has been removed from base_delivery_carrier

Author: hparfr

delivery_roulier/models/delivery_carrier.py

@@ -39,7 +39,7 @@ def cancel_shipment(self, pickings):
     # Maybe we will merge all this in future versions
     def get_tracking_link(self, picking):
         if self._is_roulier():
-            packages = picking._get_packages_from_picking()
+            packages = picking.package_ids
             first_package = packages and packages[0]
             if first_package:
                 return first_package._get_tracking_link(picking)