Re-enable moving events between collections in korganizer

Summary:
Whatever bug was happening when it was disabled, seems to be fixed by
now. The moving works well in my tests.

Test Plan:
Moving to another collection in the same DAV resource, works (graphically),
moving to another resource (icaldir), works (I can see the file on disk).

Reviewers: dvratil, vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20340