Remove salesperson reference from commission

Remove the salesperson reference from commission and use the reference the
sale has. This will fix a bug when:

1) Sale was created
2) Payments were added to the sale
3) The sales person changed

The final result was that the commission was still referencing the wrong
sales person.

Fixes: http://bugs.async.com.br/show_bug.cgi?id=5582
Change-Id: I4d2838696a8b24c49926f3ab760923de25357c80