clean up PlatformHelpers (react-navigation#4586)

this had previously been required for old versions of react native and react-native-web

Author: ericvicenti

src/PlatformHelpers.native.js

@@ -1,8 +1,7 @@
 import React from 'react';
-import { AsyncStorage, Linking, Platform } from 'react-native';
+import { AsyncStorage, Linking, Platform, BackHandler } from 'react-native';
 import { polyfill } from 'react-lifecycles-compat';
 
-import { BackHandler } from './PlatformHelpers';
 import NavigationActions from './NavigationActions';
 import getNavigation from './getNavigation';
 import invariant from './utils/invariant';

src/PlatformHelpers.web.js

@@ -9,8 +9,8 @@ import {
   View,
   I18nManager,
   ViewPropTypes,
+  MaskedViewIOS,
 } from 'react-native';
-import { MaskedViewIOS } from '../../PlatformHelpers';
 import SafeAreaView from 'react-native-safe-area-view';
 
 import HeaderTitle from './HeaderTitle';