Check for cross-signing homeserver support

This checks the homeserver to ensure it supports cross-signing (via the versions
feature flag) before attempting bootstrapping or offering user verification.

Fixes https://github.com/vector-im/riot-web/issues/11863

src/components/views/settings/KeyBackupPanel.js

@@ -127,7 +127,6 @@ export default class KeyBackupPanel extends React.PureComponent {
         Modal.createTrackedDialogAsync('Key Backup', 'Key Backup',
             import('../../../async-components/views/dialogs/keybackup/CreateKeyBackupDialog'),
             {
-                secureSecretStorage: SettingsStore.isFeatureEnabled("feature_cross_signing"),
                 onFinished: () => {
                     this._loadBackupStatus();
                 },