Kolab: fix warning "Object type not handled" for each tag/relation

Summary:
The warning turned out to be harmless, the object type
Kolab::RelationConfigurationObject is handled in KolabRetrieveTagTask.

Test Plan: syncing a kolab resource

Reviewers: cmollekopf, dvratil

Reviewed By: dvratil

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D7376