[FIX] orm: _validate should not immediately rollback

For some obscure reason _validate used to immediately
rollback the current transaction as soon as a constraint
failed anywhere. This is completely incorrect and
violates the transaction abstraction: the responsibility
of creating and closing transactions belongs to the
RPC dispatch layer, or whatever takes its place (e.g. the
test setup/teardown for tests). Rolling back or committing
in the middle of a transaction precludes special
error treatment and can have very bad side effects.

bzr revid: odo@openerp.com-20121029180742-2gw08kobdh7w5njc

openerp/osv/orm.py

@@ -1548,7 +1548,6 @@ class BaseModel(object):
                 )
                 self._invalids.update(fields)
         if error_msgs:
-            cr.rollback()
             raise except_orm('ValidateError', '\n'.join(error_msgs))
         else:
             self._invalids.clear()