Revert "[fix] the added new constraint for account (via OPW)"

This reverts commit 8e83f2af9d41e68007fcd337b697138ba237d0b0.

This has been resolved upstream: 
https://bugs.launchpad.net/openobject-addons/+bug/1215897/comments/5
https://github.com/odoo/odoo/commit/cc5d5bb2bf4a84a27edf998cee22c8602c9a78e0