Merge QmitknnUNetToolSlots.cpp & QmitknnUNetToolGUI.cpp T29205

Summary: Remove QmitknnUNetToolSlots.cpp file and migrate contents to QmitknnUNetToolGUI.cpp file. (T29205)

Test Plan: NA

Reviewers: kislinsk, O1 MITK Reviewer Group I

Reviewed By: kislinsk, O1 MITK Reviewer Group I

Differential Revision: https://phabricator.mitk.org/D679