[FIX] purchase_stock: improve partner._compute_on_time_rate perfs

Improve res.patner._compute_on_time_rate performances by calling
mapped('quantity_done') once on all the order_lines.move_ids.

Moving it out of the for loop allows to befinit from the performance
gains of the read_group call in stock.move._quantity_done_compute.

closes odoo/odoo#72068

Signed-off-by: Rémy Voet <ryv-odoo@users.noreply.github.com>