Correctly handle segmentation observers in interpolation

Summary:
Fixes the second part of T29547. Previously, new observers were always added when the working data changes, but never removed until the interpolator gets uninitialized. Now, every time the working data changes or gets deleted, any previous observers are removed.
This covers the crashes I experienced / could find. If something more was meant in T29547 that I did not fix here, please let me know.

Test Plan: Create a segmentation, press "Close project", load an image again, create new segmentation. No crashes should occur.

Reviewers: O1 MITK Reviewer Group I, kislinsk

Reviewed By: O1 MITK Reviewer Group I, kislinsk

Maniphest Tasks: T29547

Differential Revision: https://phabricator.mitk.org/D821