Bug 1598305, remove what appears to be a non-existent frame script from the test browser_no_tabs.js, r=MattN

Differential Revision: https://phabricator.services.mozilla.com/D61911

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

browser/components/uitour/test/browser_no_tabs.js

@@ -69,12 +69,6 @@ add_task(async function test_windowless_UITour() {
     browserPromise.then(frameInfo => {
       isnot(frameInfo.browser, null, "The browser must exist and not be null.");
 
-      // Load UITour frame script.
-      frameInfo.browser.messageManager.loadFrameScript(
-        "chrome://browser/content/content-UITour.js",
-        false
-      );
-
       // When the page loads, try to use UITour API.
       frameInfo.browser.addEventListener(
         "load",