Bug 1599406 - Fix permafailing browser/components/extensions/test/browser/browser_ext_urlbar.js on beta r=mixedpuppy

The `onEngagement` task fails on beta due to how the mock extension is created with `useAddonManager: "temporary"`, which prevents the extension from being able to use `browser.urlbar`. That's the only task in the file that does that. I think I copied it from another test file. Not sure why I just didn't use the pattern in this file.

This also removes `incognitoOverride: "spanning"`. It's not necessary and none of the other tasks do that either.

Differential Revision: https://phabricator.services.mozilla.com/D54795

--HG--
extra : moz-landing-system : lando

browser/components/extensions/test/browser/browser_ext_urlbar.js

@@ -456,8 +456,6 @@ add_task(async function onEngagement() {
       permissions: ["urlbar"],
     },
     isPrivileged: true,
-    incognitoOverride: "spanning",
-    useAddonManager: "temporary",
     background() {
       browser.urlbar.onEngagement.addListener(state => {
         browser.test.sendMessage("onEngagement", state);