Clean SliceNavigationController to remove unnecessary data members / member functions

Summary:
Remove unexpected side-effect

The default-parameter was set to true and thus using the connect
functions led to another geometry event silently being connected as well.

Clean includes

Remove namespace prefix

Use auto

Apply clang format

Manual formatting

Clean code, consistent style, early return.

Remove unnecessary / unused functions / variable

These functions and data members are not used and can safely be removed.

Test Plan: check CI builds

Reviewers: kislinsk, O3 MITK Reviewer Group II, O1 MITK Reviewer Group I

Reviewed By: kislinsk, O3 MITK Reviewer Group II, O1 MITK Reviewer Group I

Subscribers: kalali, kislinsk

Maniphest Tasks: T29266

Differential Revision: https://phabricator.mitk.org/D702