[FIX] point_of_sale: apply cogs on delivery picking only

Commit 1e82e273 adds cogs account move lines for 'ship later'
config at the picking validation. The issue appears if the delivery flow
is in multiple steps. The account move lines will be created for each
pickings.

This commit ensure the last one actually create the aml only

closes odoo/odoo#125358

Opw: 3324972
X-original-commit: b7e3db6f16e0ce42b226fecce05dfa5b586190fd
Signed-off-by: William Henrotin (whe) <whe@odoo.com>