Fix connecting of OnCurrentSelectionChanged to signal

Summary: The receiver of the CurrentSelectionChanged signal needs to be connected before QmitkSingleNodeSelectionWidget->SetAutoSelectNewNodes(true) is set, otherwise the initial auto select gets lost. Thus, I removed an early duplicate call to this method which is called again at the end, because once it's set it isn't updated by another call. Fixes T28823

Test Plan: See T28823, should work directly now

Reviewers: O3 MITK Reviewer Group II, kislinsk

Reviewed By: O3 MITK Reviewer Group II, kislinsk

Subscribers: kislinsk, kalali

Maniphest Tasks: T28823

Differential Revision: https://phabricator.mitk.org/D564