Bugfix: Local logout if IdP has no logout support

Some IdPs don't support a logout endpoint. In those cases the web UI ran into a fatal error and showed an empty screen without
further redirects. Fixed by forgetting the currently authenticated user when the OpenID Connect configuration doesn't contain
an `endSessionEndpoint` url.

https://github.com/owncloud/web/pull/10974
https://github.com/owncloud/enterprise/issues/6631
