Disable key request dialogs with cross-signing

Cross-signing verification is meant to replace the old key share between devices
flow. This disables it when the cross-signing lab is enabled.

Fixes https://github.com/vector-im/riot-web/issues/11904

src/components/structures/MatrixChat.js

@@ -1383,6 +1383,8 @@ export default createReactClass({
         cli.on("Session.logged_out", () => dft.stop());
         cli.on("Event.decrypted", (e, err) => dft.eventDecrypted(e, err));
 
+        // TODO: We can remove this once cross-signing is the only way.
+        // https://github.com/vector-im/riot-web/issues/11908
         const krh = new KeyRequestHandler(cli);
         cli.on("crypto.roomKeyRequest", (req) => {
             krh.handleKeyRequest(req);