Bug 1765029 - devtools/docs: remove an old link r=nchevobbe

Differential Revision: https://phabricator.services.mozilla.com/D152085

devtools/docs/contributor/contributing/css.md

@@ -81,7 +81,6 @@ It's recommended to use SVG since it keeps the CSS clean when supporting multipl
 
 ## Performance
 
-* Read [Writing Efficient CSS](https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Writing_efficient_CSS).
 * Use an iframe where possible so your rules are scoped to the smallest possible set of nodes.<!--TODO: is this still true? and also refine exactly when it is appropriate to use an iframe. Examples might help-->
 * If your CSS is used in `browser.xhtml`, you need to take special care with performance:
  * Descendent selectors should be avoided.