Drop grey icon themes

[vkbo]

Summary:

This PR deletes the Typicons Grey themes. These icons are no longer as useful after the recent layout changes. Coloured icons are now needed to show the difference between document types.

Related Issue(s):

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed