Remove CSS for nprogress removed

What does this MR do?

Removes unused CSS for nprogress forgotten to remove when it was removed (!3004 (merged)).

Are there points in the code the reviewer needs to double check?

No.

Note I grepped [^i]nprogress through whole code to check if not used.

cf. https://github.com/rstacruz/nprogress/blob/d68df8d5608112d6c8abb5658fa5515471eaaabc/nprogress.css

Why was this MR needed?

To keep code clean and compact. This is also the first stage of preparation for IdSelector rule of scss-lint (#23485 (moved)).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#23485 (moved)