[FIX] mass_mailing: fix copy of a `mailing.mailing`
[FIX] mass_mailing: fix copy of a `mailing.mailing`

Bug
===
In ffe1d4047da181f97fafe2a69f64e6f283fbeb0e "onchange" were replaced
by computed fields. But in 950296f661499f499b0a5776ce7fd91a66c2a63f,
they use manually this onchange method in the copy, which is not needed
with computed field (bug introduced certainly during a forward port).

Task-2226235

closes odoo/odoo#48531

X-original-commit: 63712e106dfae6ef99d93a0ba2ff4102ed430719
Signed-off-by: Thibault Delavallee (tde) <2d325771bb6e8870d87f0f3f7b45589a8338e550@openerp.com>

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo