user_saml: Fix slo initiated by owncloud

SimpleSAML need SimpleSAMLAuthToken cookie to proceed a correct logout

When you remove the SimpleSAMLAuthToken cookie before a SimpleSAML logout. SimpleSAML traceback with this error : "SimpleSAML_Error_NoState: NOSTATE" during the slo.
To fix this issue we need to keep this cookie and the SimpleSAML's logout method will remove it.