Revert "[FIX] purchase: update order_lines date_planned"
Revert "[FIX] purchase: update order_lines date_planned"

This reverts commit dcf50bf2422b242c96d6b1ff33236fa0e56335ae.

The commit should not have been forward-ported after 9.0 since
Model update method has been @api.multi'ed.

This also introduced an issue since changing the first line would
change the date_planned of the order (which would alter subesquent
updates).

opw-697680

dfang/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,hip-odoo/odoo,dfang/odoo,dfang/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,ygol/odoo