ui: Remove warning for unsupported old Zulip desktop app.

This warning was added in #6551.  It’s not for any version of the
current Electron app, which we warn about on the server side with
DESKTOP_WARNING_VERSION, but rather some pre-Electron app so ancient I
don’t even know what it is.  Apparently it communicated using the
window.bridge global, so eradicate that too.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

frontend_tests/node_tests/compose.js

@@ -13,8 +13,6 @@ const $ = require("../zjsunit/zjquery");
 
 const events = require("./lib/events");
 
-set_global("bridge", false);
-
 const noop = () => {};
 
 set_global("DOMParser", new JSDOM().window.DOMParser);
@@ -1029,7 +1027,6 @@ test_ui("initialize", () => {
         assert.equal("#compose-textarea", elem);
         resize_watch_manual_resize_checked = true;
     };
-    set_global("bridge", true);
     let xmlhttprequest_checked = false;
     set_global("XMLHttpRequest", function () {
         this.upload = true;