The buildin IDP doesn't honor `OCIS_LDAP_DISABLE_USER_MECHANISM`

## Describe the bug

The builtin IDP doesn't honor `OCIS_LDAP_DISABLE_USER_MECHANISM`. So `OCIS_LDAP_DISABLE_USER_MECHANISM=none` does not work when using the internal IDP. It defaults to the `OCIS_LDAP_DISABLE_USER_MECHANISM=attribute`. Which makes integration with external LDAP servers somewhat tricky. It's possible to workaround this by settings `OCIS_LDAP_USER_ENABLED_ATTRIBUTE=""` but that's inconsitent with how the other service are configured.

The IDP should at least support `none` and `attribute` for `OCIS_LDAP_DISABLE_USER_MECHANISM`. I think we should not implement `OCIS_LDAP_DISABLE_USER_MECHANISM=group` in the IDP for now. (If anything we should error out if `OCIS_LDAP_DISABLE_USER_MECHANISM` set to something else than `none` or `attribute`.

This is a follow up to https://github.com/owncloud/ocis/issues/6513