DNM - Delete `CSS_key_concepts` macro

[SphinxKnight]

Do not merge yet Prerequisites:

• Blocking: https://github.com/mdn/translated-content/issues/7343
• Non-blocking: https://github.com/mdn/yari/pull/6819

See also: https://github.com/mdn/content/pull/19105

Summary

Removes another macro used to generate simple links

Problem

KS old technical debt

Solution

Remove the macro altogether

How did you test this change?

I didn't

[github-actions[bot]]

This pull request has merge conflicts that must be resolved before it can be merged.

[caugner]

The macro is still used several times in 3 locales (see here), so I will close this PR for now. Feel free to reopen or ping me here if the macro is no longer used. 😉