[FIX] purchase: remove unused search query

Search result is not used since switching to implementation with `read_group`.
The line is already removed in v14, but because it may significantly slow down
computation of the `purchased_product_qty` field, we should remove it in v13 too

opw-2971825

closes odoo/odoo#99800

Signed-off-by: Adrien Widart <awt@odoo.com>