Use single-spa-react's default domElementGetter

[Joel Denning]

In recent versions of single-spa-react, you do not need to provide a domElementGetter.

See https://github.com/crossjs/spax/blob/master/packages/demo-single-spa/src/app2/app2.tsx#L25, which can be deleted entirely

// No need to provide domElementGetter anymore.
const reactLifecycles = const reactLifecycles = Root.then((R) => singleSpaReact({
  React,
  ReactDOM,
  rootComponent: () => R,
}));