Remove unused QmitkSelectableGLWidget

Summary: Remove QmitkSelectableGLWidget and unnecessary includes

Test Plan: build without compile errors

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

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

Subscribers: kalali, kislinsk

Maniphest Tasks: T29265

Differential Revision: https://phabricator.mitk.org/D717