inventory: always fetch tray

The check was means as an optimization: no need to fetch at tray already
open. But "fetch_tray" will not only open the tray, it may also move the
laser on the exact position. So  we should do it for every inventory line.

stock_vertical_lift/models/vertical_lift_operation_inventory.py

@@ -232,17 +232,13 @@ def fetch_tray(self):
 
     def select_next_inventory_line(self):
         self.ensure_one()
-        previous_line = self.current_inventory_line_id
         next_line = self.env["stock.inventory.line"].search(
             self._domain_inventory_lines_to_do(),
             limit=1,
             order="vertical_lift_tray_id, location_id, id",
         )
         self.current_inventory_line_id = next_line
-        if (
-            next_line
-            and previous_line.vertical_lift_tray_id != next_line.vertical_lift_tray_id
-        ):
+        if next_line:
             self.fetch_tray()
         return bool(next_line)