fix(css): remove the unimplemented src()

[OnkarRuikar]

• fix https://github.com/mdn/content/issues/36831
• related to https://github.com/mdn/browser-compat-data/pull/25081#pullrequestreview-2439709005

Per our policy, we don't document stuff that is not implemented in browsers.

[github-actions[bot]]

Preview URLs

• /en-US/docs/Web/CSS/At-rule-functions
• /en-US/docs/Web/CSS/url_value