remove onchange picking type on purchase order

The removed code changed the destination address of the PO with the
address of the warehouse related to the picking type. That is bad for
two reasons:

- The dest_address_id is used for the customer address in dropshipping
  scenarios. That has nothing to do with the warehouse address.
- The dest_address_id has itself an onchange that sets the destination
  address to customers, because we are in a dropshipping scenario. Thus,
  this commit fixes #56.

I didn't manage to write a test for that because of odoo/odoo#3503.