Remove BackAndroid, which has had a deprecation warning (#21866)

Summary:
Remove BackAndroid, which has had a deprecation warning and only forwarded to BackHandler since March 2018.

Test Plan
---------
React-native init bundle and RNTester bundle works.

Release Notes:
--------------
[ ANDROID  ] [ BREAKING ] [ BackAndroid ] - Deprecate BackAndroid since BackHandler should be used in its place.
Pull Request resolved: https://github.com/facebook/react-native/pull/21866

Differential Revision: D10472419

Pulled By: TheSavior

fbshipit-source-id: 3d76e1ce4c74bb783fee7fd8232bb366f2e7ea12

Libraries/react-native/react-native-implementation.js

@@ -195,9 +195,6 @@ module.exports = {
   get AsyncStorage() {
     return require('AsyncStorage');
   },
-  get BackAndroid() {
-    return require('BackAndroid');
-  }, // deprecated: use BackHandler instead
   get BackHandler() {
     return require('BackHandler');
   },