Replace deprecated Linking.removeEventListener(), to remove() , throwing error undefined is not a function

Hi! 👋

Firstly, thanks for your work on this project! 🙂

Today I used [patch-package](https://github.com/ds300/patch-package) to patch `react-native-router-flux@4.3.1` for the project I'm working on.

Here is the diff that solved my problem:

```diff
diff --git a/node_modules/react-native-router-flux/src/Router.js b/node_modules/react-native-router-flux/src/Router.js
index 6f595e4..c9baf20 100644
--- a/node_modules/react-native-router-flux/src/Router.js
+++ b/node_modules/react-native-router-flux/src/Router.js
@@ -6,6 +6,8 @@ import defaultStore from './defaultStore';
import pathParser from './pathParser';

class App extends React.Component {
+ hardwareBackPressSubscriber ;
+ urlSubscriber;
static propTypes = {
navigator: PropTypes.func,
backAndroidHandler: PropTypes.func,
@@ -22,17 +24,17 @@ class App extends React.Component {
};

componentDidMount() {
- BackHandler.addEventListener('hardwareBackPress', this.props.backAndroidHandler || this.onBackPress);
+ this.hardwareBackPressSubscriber = BackHandler.addEventListener('hardwareBackPress', this.props.backAndroidHandler || this.onBackPress);

// If the app was "woken up" by an external route.
Linking.getInitialURL().then(url => this.parseDeepURL(url));
// Add an event listener for further deep linking.
- Linking.addEventListener('url', this.handleDeepURL);
+ this.urlSubscriber = Linking.addEventListener('url', this.handleDeepURL);
}

componentWillUnmount() {
- BackHandler.removeEventListener('hardwareBackPress', this.props.backAndroidHandler || this.onBackPress);
- Linking.removeEventListener('url', this.handleDeepURL);
+ this.hardwareBackPressSubscriber.remove();
+ this.urlSubscriber.remove();
}

onBackPress = () => this.props.navigationStore.pop();
```

This issue body was [partially generated by patch-package](https://github.com/ds300/patch-package/issues/296).