[FIX] point_of_sale: allow opening the pos interface

The `cash_control` constraint [1] is only relevant when the field
`cash_control` is modified. Do not check it when opening the pos UI.

[1] added by 733fb360

addons/point_of_sale/models/pos_config.py

@@ -461,7 +461,7 @@ class PosConfig(models.Model):
         """ open the pos interface """
         self.ensure_one()
         # check all constraints, raises if any is not met
-        self._validate_fields(self._fields)
+        self._validate_fields(set(self._fields) - {"cash_control"})
         return {
             'type': 'ir.actions.act_url',
             'url':   '/pos/web/',