[IMP] hr_expense: less email notification on assignation

Managers were receiving 2 emails to say basically the same thing. This
commit removes one of those emails ("You have been assigned to ...").

closes odoo/odoo#39058

Taskid: 2067994
X-original-commit: 1dbffeb7dba9ed862d6c62ebd771e7581dd33250
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>