[FIX] mail: wrong many2many definition

The definition of `attachment_ids` on model `email_template.preview` is wrong,
because its table/columns refer to the model `mail.template`.  As the field is
only used to preview a result in a wizard form, it does not need to be stored.

closes odoo/odoo#29349

addons/mail/wizard/email_template_preview.py

@@ -34,6 +34,7 @@ class TemplatePreview(models.TransientModel):
 
     res_id = fields.Selection(_get_records, 'Sample Document')
     partner_ids = fields.Many2many('res.partner', string='Recipients')
+    attachment_ids = fields.Many2many(string='Attachments', store=False)
 
     @api.onchange('res_id')
     @api.multi

odoo/fields.py

@@ -2329,6 +2329,8 @@ class Many2many(_RelationalMulti):
                     self.column2 = '%s_id' % comodel._table
             # check validity of table name
             check_pg_name(self.relation)
+        else:
+            self.relation = self.column1 = self.column2 = None
 
     def _setup_regular_full(self, model):
         super(Many2many, self)._setup_regular_full(model)