[FIX] product, sale, stock: speed up discarding product variants

Modifying product attributes involves discarding product variants via
the method _unlink_or_archive(), which tries to delete variants, and
archive the ones for which the deletion fails.  This process is very
slow when most product variants must be archived.  Instead, we filter
out the variants that must be archived anyway (like the ones used in
sales or stock), in order to avoid the variants that cannot be deleted.

On a product with ~200 variants, the time goes from more than 15 minutes
(the request times out) to around 5 seconds!

opw-2440417

closes odoo/odoo#65024

X-original-commit: 63b2f850
Signed-off-by: Raphael Collet (rco) <rco@openerp.com>