Fix warning when opening volume visualization

Summary: Small refactoring. The m_RangeSliderMax and m_RangeSliderMin are uneccesary, since the information can be directly get from the m_RangeSlider itself. This also fixes the warning that pops up when the volume visualization is opened with any image loaded.

Test Plan: load an image, open volume visualization. In the log, there should be no warning anymore (and the volume visualization should work as usual, of course)

Reviewers: O3 MITK Reviewer Group II, kislinsk

Reviewed By: O3 MITK Reviewer Group II, kislinsk

Differential Revision: https://phabricator.mitk.org/D957