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/views/dialogs/KeyShareDialog.js

@@ -22,6 +22,9 @@ import * as sdk from '../../../index';
 
 import { _t, _td } from '../../../languageHandler';
 
+// TODO: We can remove this once cross-signing is the only way.
+// https://github.com/vector-im/riot-web/issues/11908
+
 /**
  * Dialog which asks the user whether they want to share their keys with
  * an unverified device.