Removed old MaterialLib code

Christoph Lehmannrequested to merge
chleh/ogs:improve-docu-qa into master

This MR removes some obsolete code in the MaterialLib, thereby reducing the number of issues on the Doxygen docu QA page.

  1. Feature description was added to the changelog
  2. Tests covering your feature were added?
  3. Any new feature or behaviour change was documented?
Edited by Christoph Lehmann

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