Removing css tool link

[digi-booster]

As @rachelandrew removed the CSS tool's page #8522 , there is no need to link to that page anymore.

Issue number that this PR fixes (if any). For example: 'Fixes #987654321'

What was wrong/why is this fix needed? (quick summary only)

Anything else that could help us review it

[github-actions]

Preview URLs

• https://pr8547.content.dev.mdn.mozit.cloud/en-US/docs/Web/CSS

Flaws

None! 🎉

External URLs

URL: /en-US/docs/Web/CSS
Title: CSS: Cascading Style Sheets
on GitHub

• https://addons.mozilla.org/en-US/firefox/addon/web-developer/ (1 time)
• https://bugzilla.mozilla.org/show_bug.cgi?id=1323667 (1 time)
• https://jigsaw.w3.org/css-validator/ (1 time)
• https://stackoverflow.com/questions/tagged/css (1 time)
• https://w3.org/Style/CSS/ (1 time)
• https://www.w3.org/Style/CSS/current-work (1 time)
• https://www.w3.org/TR/css/ (1 time)

[wbamberg]

👍 nice catch, thank you!