Bug 1857565 - Remove leftover -moz-image-rect from function-no-unknown lint. r=desktop-theme-reviewers,dao

It's gone.

Differential Revision: https://phabricator.services.mozilla.com/D190312

.stylelintrc.js

@@ -47,7 +47,6 @@ module.exports = {
       {
         ignoreFunctions: [
           "light-dark" /* Used for color-scheme dependent colors */,
-          "-moz-image-rect" /* Used for cropping images */,
           "add" /* Used in mathml.css */,
         ],
       },