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/Utilities/BackAndroid.js

@@ -1,54 +0,0 @@
-/**
- * Copyright (c) Facebook, Inc. and its affiliates.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- *
- * BackAndroid has been moved to BackHandler.  This stub calls BackHandler methods
- * after generating a warning to remind users to move to the new BackHandler module.
- *
- * @format
- */
-
-'use strict';
-
-const BackHandler = require('BackHandler');
-
-const warning = require('fbjs/lib/warning');
-
-/**
- * Deprecated.  Use BackHandler instead.
- */
-const BackAndroid = {
-  exitApp: function() {
-    warning(
-      false,
-      'BackAndroid is deprecated.  Please use BackHandler instead.',
-    );
-    BackHandler.exitApp();
-  },
-
-  addEventListener: function(
-    eventName: BackPressEventName,
-    handler: Function,
-  ): {remove: () => void} {
-    warning(
-      false,
-      'BackAndroid is deprecated.  Please use BackHandler instead.',
-    );
-    return BackHandler.addEventListener(eventName, handler);
-  },
-
-  removeEventListener: function(
-    eventName: BackPressEventName,
-    handler: Function,
-  ): void {
-    warning(
-      false,
-      'BackAndroid is deprecated.  Please use BackHandler instead.',
-    );
-    BackHandler.removeEventListener(eventName, handler);
-  },
-};
-
-module.exports = BackAndroid;

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');
   },