[FIX] Make sure we create a new partner on import.

When we try to import a funtion partner with a cantact_id field pointing to an existing contact, the create method will just return the existing contact id instead of creating a new entry in the database but the import will think that the importation went ok since no exception is triggered.

This PR removes the code that causes this behaviour.

The CVS importing routine sets the install_mode on triggering this behaviour.