rely only on php DateTime to parse the db datetime string

Fixes https://github.com/owncloud/core/issues/6327

The whole conversion to a DateTime object via Doctrine seems unneccessary because only SQLServer uses a different datetime format. The default assumed by doctrine is Y-m-d H:i:s and SQLServer / SQLServer2008 uses Y-m-d H:i:s.000 / Y-m-d H:i:s.u. phps DateTime very well understands any of these formats. So why bother using doctrine here at all?

@bartv2 @karlitschek @PVince81