[FIX] core,fleet: remove group_operator for Many2oneReference

The default `group_operator` of `Many2oneReference` is 'sum' (inherited
from the parent class `Integer`), it doesn't make any functional sense
to sum ids.

Also set group_operator to None on `co2` instead of override read_group
for the same result.

Part-of: odoo/odoo#110737

addons/fleet/models/fleet_vehicle.py

@@ -88,7 +88,7 @@ class FleetVehicle(models.Model):
     horsepower = fields.Integer(compute='_compute_model_fields', store=True, readonly=False)
     horsepower_tax = fields.Float('Horsepower Taxation', compute='_compute_model_fields', store=True, readonly=False)
     power = fields.Integer('Power', help='Power in kW of the vehicle', compute='_compute_model_fields', store=True, readonly=False)
-    co2 = fields.Float('CO2 Emissions', help='CO2 emissions of the vehicle', compute='_compute_model_fields', store=True, readonly=False, tracking=True)
+    co2 = fields.Float('CO2 Emissions', help='CO2 emissions of the vehicle', compute='_compute_model_fields', store=True, readonly=False, tracking=True, group_operator=None)
     co2_standard = fields.Char(compute='_compute_model_fields', store=True, readonly=False)
     category_id = fields.Many2one('fleet.vehicle.model.category', 'Category', compute='_compute_model_fields', store=True, readonly=False)
     image_128 = fields.Image(related='model_id.image_128', readonly=True)
@@ -356,12 +356,6 @@ class FleetVehicle(models.Model):
     def _read_group_stage_ids(self, stages, domain, order):
         return self.env['fleet.vehicle.state'].search([], order=order)
 
-    @api.model
-    def read_group(self, domain, fields, groupby, offset=0, limit=None, orderby=False, lazy=True):
-        if 'co2' in fields:
-            fields.remove('co2')
-        return super(FleetVehicle, self).read_group(domain, fields, groupby, offset, limit, orderby, lazy)
-
     def return_action_to_open(self):
         """ This opens the xml view specified in xml_id for the current vehicle """
         self.ensure_one()

odoo/fields.py

@@ -3072,6 +3072,7 @@ class Many2oneReference(Integer):
     type = 'many2one_reference'
 
     model_field = None
+    group_operator = None
 
     _related_model_field = property(attrgetter('model_field'))