[FIX] sale: fix header in quotation mail

A header in the mail gives the amount and the validity date of a
quotation. Its means the quotation will expire at this date, and is not
related to any missing payment. The validity date has no meaning on a
confirmed sale order and is therefore removed.

task-3185072

closes odoo/odoo#115323

Signed-off-by: Vallaeys Valentin (vava) <vava@odoo.com>