[IMP] stock: improve stock_inventory action_start perfs

Backport branch odoo/odoo#79587 to v13.

In _get_inventory_lines_values, remove quants recordset
as it has been unused since commit cc9324f7.

Browse product.product in _get_inventory_lines_values
outside of for loop to speedup getting the uom_id.

closes odoo/odoo#80106

Signed-off-by: William Henrotin (whe) <whe@odoo.com>