chore: remove unnecessary mutationobserver-shim

jsdom 13.2.0 added direct support for MutationObserver, and recommends
removing all workarounds. We're on version 19.0.

Refs: jsdom/jsdom#639 (comment)

Signed-off-by: Mike Fiedler <miketheman@gmail.com>

package.json

@@ -59,7 +59,6 @@
     "jest": "28.1.0",
     "jest-environment-jsdom": "^28.1.0",
     "jest-fetch-mock": "2.1.2",
-    "mutationobserver-shim": "0.3.3",
     "sass-lint": "1.13.1"
   },
   "eslintConfig": {

tests/frontend/setup.js

@@ -11,26 +11,6 @@
  * limitations under the License.
  */
 
-// Setup MutationObserver shim since jsdom doesn't
-// support it out of the box.
-
-const fs = require("fs");
-const path = require("path");
-
-const shim = fs.readFileSync(
-  path.resolve(
-    "node_modules",
-    "mutationobserver-shim",
-    "dist",
-    "mutationobserver.min.js"
-  ),
-  { encoding: "utf-8" }
-);
-const script = window.document.createElement("script");
-script.textContent = shim;
-
-window.document.body.appendChild(script);
-
 // Extend Jest with jest-dom https://github.com/testing-library/jest-dom
 import "@testing-library/jest-dom/extend-expect";