[FIX] sale: always set fiscal position

An issue occurs in the following case:
- create a SO
- set a partner for which a fiscal position applies
- change the partner to a partner for which no fiscal position applies

The fiscal position set in a first place remains. This is a major issue
when interacting with the eCommerce, since a fiscal position can be
first set based on the GeoIP results, then might need to be removed when
the user fills in his address details.

opw-689281