Remove threads labs flag and the ability to disable threads (#9878)

cypress/e2e/polls/polls.spec.ts

@@ -1,5 +1,5 @@
 /*
-Copyright 2022 The Matrix.org Foundation C.I.C.
+Copyright 2022 - 2023 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -83,7 +83,6 @@ describe("Polls", () => {
     };
 
     beforeEach(() => {
-        cy.enableLabsFeature("feature_threadenabled");
         cy.window().then((win) => {
             win.localStorage.setItem("mx_lhs_size", "0"); // Collapse left panel for these tests
         });