Bug 1675248 - [marionette] Don't wait for Marionette:ListenersAttached in openTab(). r=marionette-reviewers,maja_zf

This code doesn't seem to be necessary anymore, and can be removed.

Differential Revision: https://phabricator.services.mozilla.com/D96111

testing/marionette/browser.js

@@ -390,32 +390,12 @@ browser.Context = class {
 
     switch (this.driver.appName) {
       case "firefox":
-        const loaded = [waitForEvent(this.window, "TabOpen")];
-
+        const opened = waitForEvent(this.window, "TabOpen");
         this.window.BrowserOpenTab();
+        await opened;
+
         tab = this.tabBrowser.selectedTab;
 
-        // wait until the framescript has been registered
-        loaded.push(
-          new Promise(resolve => {
-            let cb = msg => {
-              if (msg.json.frameId === tab.linkedBrowser.browsingContext.id) {
-                this.driver.mm.removeMessageListener(
-                  "Marionette:ListenersAttached",
-                  cb
-                );
-                resolve();
-              }
-            };
-            this.driver.mm.addMessageListener(
-              "Marionette:ListenersAttached",
-              cb
-            );
-          })
-        );
-
-        await Promise.all(loaded);
-
         // The new tab is always selected by default. If focus is not wanted,
         // the previously tab needs to be selected again.
         if (!focus) {