[IMP] ir.ui.view: discard custom views before updating view arch

- as the custom views are still in place when validating the new arch, they would
hide the new arch and prevent proper validation (if custom views are set for uid 1)
- conversely, if the RNG validation has changed, the old view customizations may not
pass the updated validation rules, and cause spurious validation errors

bzr revid: odo@openerp.com-20120924104026-z7bjzzq80ifxy1oc