BackNavigationHandler: Delete, as handled naturally.

Some notable commits in the history of this logic:

0688b9b Handle Android hardware back button press. (#1125)
6785245 Improve and fix Back button on Android
ef99027 navigation: Split AppWithNavigation

Nothing suggests that we've been trying to do anything more
complicated than the following default behavior of React Navigation
(in the assumed absence of react-navigation-redux-helpers, which we
removed in a recent commit) [1]:

"""
By default, when user presses the Android hardware back button,
react-navigation will pop a screen or exit the app if there are no
screens to pop. This is a sensible default behavior, but there are
situations when you might want to implement custom handling.
"""

Logic like what we remove here was probably necessary when using
react-navigation-redux-helpers; that package gives an example [2]
that looks suspiciously similar to what we've been using. So, to be
safe, we don't remove until here, after we've definitely stopped
using r-n-r-h.

[1] https://reactnavigation.org/docs/4.x/custom-android-back-button-handling/
[2] https://github.com/react-navigation/redux-helpers#back-button

Author: chrisbobbe

src/ZulipMobile.js

@@ -9,7 +9,6 @@ import ThemeProvider from './boot/ThemeProvider';
 import CompatibilityChecker from './boot/CompatibilityChecker';
 import AppEventHandlers from './boot/AppEventHandlers';
 import AppDataFetcher from './boot/AppDataFetcher';
-import BackNavigationHandler from './nav/BackNavigationHandler';
 import InitialNavigationDispatcher from './nav/InitialNavigationDispatcher';
 import AppContainer from './nav/AppContainer';
 import NavigationService from './nav/NavigationService';
@@ -30,14 +29,12 @@ export default (): React$Node => (
           <TranslationProvider>
             <ThemeProvider>
               <InitialNavigationDispatcher>
-                <BackNavigationHandler>
-                  <AppContainer
-                    // `static navigationOptions` and `static router` not
-                    // being handled properly
-                    // $FlowFixMe
-                    ref={NavigationService.appContainerRef}
-                  />
-                </BackNavigationHandler>
+                <AppContainer
+                  // `static navigationOptions` and `static router` not
+                  // being handled properly
+                  // $FlowFixMe
+                  ref={NavigationService.appContainerRef}
+                />
               </InitialNavigationDispatcher>
             </ThemeProvider>
           </TranslationProvider>