clean up code by removing test selector (#22520)

ui/lib/core/addon/components/json-editor.hbs

@@ -43,7 +43,6 @@
     }}
     class={{if @readOnly "readonly-codemirror"}}
     data-test-component="code-mirror-modifier"
-    data-test-example-modal-json-text
   ></div>
 
   {{#if @helpText}}

ui/tests/integration/components/policy-example-test.js

@@ -11,7 +11,7 @@ import { hbs } from 'ember-cli-htmlbars';
 const SELECTORS = {
   policyText: '[data-test-modal-title]',
   policyDescription: (type) => `[data-test-example-modal-text=${type}]`,
-  jsonText: '[data-test-example-modal-json-text]',
+  jsonText: '[data-test-component="code-mirror-modifier"]',
   informationLink: '[data-test-example-modal-information-link]',
 };