[FIX] sale: small cleanup of line computations

Initially a forward-port of 5aecd365, which needed f407e60a to work.

The do_in_onchange was removed inbetween, but it is not needed anymore here.
The issue was caused by the fact that the cache values given for each
environment, whereas in the new ORM the values are now shared across all.

The other potential issue caught by this test was that the company of the taxes
could be incorrect. This is problematic for the end user, since relational that
violates inter-company rules mean that they won't be able to read or write
the records without sudo. However the model now has _check_company_auto,
handled by the ORM, so the issue should not arise anymore.

closes odoo/odoo#38543

X-original-commit: 30cf4b37
Signed-off-by: Nans Lefebvre (len) <len@odoo.com>