[FIX] fleet: fix missing compute method

The PR #75475 removed the method `_compute_manager_id` without removing
the reference in the field.

closes odoo/odoo#81766

X-original-commit: a4e1477d120c336ace405421ddb860a72cfeafe1
Signed-off-by: Kevin Baptiste <kba@odoo.com>

addons/fleet/models/fleet_vehicle.py

@@ -31,7 +31,6 @@ class FleetVehicle(models.Model):
     active = fields.Boolean('Active', default=True, tracking=True)
     manager_id = fields.Many2one(
         'res.users', 'Fleet Manager',
-        compute='_compute_manager_id', store=True, readonly=False,
         domain=lambda self: [('groups_id', 'in', self.env.ref('fleet.fleet_group_manager').id)],
     )
     company_id = fields.Many2one(