Use Vanilla JS in the New App Template

Summary:
Flow [doesn't target OSS](https://medium.com/flow-type/clarity-on-flows-direction-and-open-source-engagement-e721a4eb4d8b), and using it in the new app template leads to a worse experience for anyone using VS Code.

Beyond that, the template package.json doesn't include `flow-bin` or a script to do typechecking.

Some two year old context here: react-native-community/discussions-and-proposals#296

This removes Flow annotations, flow config, and buck config (I don't immediately see BUCK used either) from the default template.

Changelog:
[General][Changed] - Use Vanilla JS in the App Template

Differential Revision: D40911951

fbshipit-source-id: f8a08ed0bb5be24996235c7dd9ffad01b4ad6636

template/App.js

@@ -3,11 +3,9 @@
  * https://github.com/facebook/react-native
  *
  * @format
- * @flow strict-local
  */
 
 import React from 'react';
-import type {Node} from 'react';
 import {
   SafeAreaView,
   ScrollView,
@@ -26,12 +24,7 @@ import {
   ReloadInstructions,
 } from 'react-native/Libraries/NewAppScreen';
 
-type SectionProps = {
-  title: string,
-  children: Node,
-};
-
-function Section({children, title}: SectionProps): Node {
+function Section({children, title}) {
   const isDarkMode = useColorScheme() === 'dark';
   return (
     <View style={styles.sectionContainer}>
@@ -57,7 +50,7 @@ function Section({children, title}: SectionProps): Node {
   );
 }
 
-function App(): Node {
+function App() {
   const isDarkMode = useColorScheme() === 'dark';
 
   const backgroundStyle = {