[FIX] purchase: replace company_id widget selection by option no_create

For selection fields, name_get calls to resolve the display name are client-side initiated, and will display "Unknown" if the user does not have read access to the selected company. And the reason for using a selection widget in the first place was to prevent inadvertent creation of companies. This is now doable via the no_create option, so we can remove the selection widget.