[FIX] base: unlinking ir.model.constraint

When unlinking an ir.model, all the ir.model.constraint linked to this
ir.model must be removed to avoid to call unexisting model.

The ir.model.constraint records, are created with module = the module where the related field
has been declared. But when unlinking the ir.model in function _module_data_uninstall, the related
ir.model.constraint to remove were chosen according to the removed modules. So with models
declaring constraints on inherited fields, some problems occured because the model could be removed
and not all the related constraints.

opw:716032

odoo/addons/base/ir/ir_model.py

@@ -167,9 +167,6 @@ class IrModel(models.Model):
                 # prevent screwing up fields that depend on these models' fields
                 model.field_id._prepare_update()
 
-        imc = self.env['ir.model.constraint'].search([('model', 'in', self.ids)])
-        imc.unlink()
-
         self._drop_table()
         res = super(IrModel, self).unlink()
 
@@ -811,7 +808,7 @@ class IrModelConstraint(models.Model):
     name = fields.Char(string='Constraint', required=True, index=True,
                        help="PostgreSQL constraint or foreign key name.")
     definition = fields.Char(help="PostgreSQL constraint definition")
-    model = fields.Many2one('ir.model', required=True, index=True)
+    model = fields.Many2one('ir.model', required=True, ondelete="cascade", index=True)
     module = fields.Many2one('ir.module.module', required=True, index=True)
     type = fields.Char(string='Constraint Type', required=True, size=1, index=True,
                        help="Type of the constraint: `f` for a foreign key, "