[FIX] payment: remove transaction from session

Sequentially previewing and paying two sale orders was causing a
redirection issue. Instead of being redirect to the SO preview
(/my/orders/<:order_id>), the user was redirected to /payment/process.

The issue was due to processed transactions not being removed from the
session.

opw-1948288
Closes #31741

Signed-off-by: Toufik Benjaa (tbe) <tbe@odoo.com>