[FIX] account_voucher: unnecessary copy of payment memo

Before this commit, the payment memo was copied during the duplication
of a voucher. The issue is that the payment memo is not always shown in
the view, and it's mostly unique by voucher.

Now, the payment memo it's not copied anymore.

opw-1986544

closes odoo/odoo#33708

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>