Remove unused assets (image files)

[schoinh]

We used these image files when we used to hard-code images for workflow buttons and diagrams. We don't need them anymore. Removing folders assets/workflow_diagrams and assets_workflow_images.

Pull request recommendations:

• [x] Name your pull request your-development-type/short-description. Ex: feature/read-tiff-files
• [ ] Link to any relevant issue in the PR description. Ex: Resolves [gh-12], adds tiff file format support
• [x] Provide context of changes.
• [ ] Provide relevant tests for your feature or bug fix.
• [ ] Provide or update documentation for any feature added by your pull request.

Thanks for contributing!

[codecov[bot]]

Codecov Report

Merging #81 (28da574) into main (4460889) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #81   +/-   ##
=======================================
  Coverage   92.78%   92.78%           
=======================================
  Files          29       29           
  Lines        1025     1025           
=======================================
  Hits          951      951           
  Misses         74       74           

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