[FIX] mail : Remove double signature on invoice email

Current behavior :
When sending an invoice by email there was 2 signature in the mail

Steps to reproduce :
Create an invoice
Send it by email
Check the mail, there are 2 signatures

opw-2703272

closes odoo/odoo#81086

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>