android deps: Finish removal of react-native-orientation.

In 0d5a19a, for zulip#4147, we forgot to remove some Android
configuration specific to `react-native-orientation` [1], added in
3149c9d. So, remove it.

It appears that the iOS configuration at the
`react-native-orientation` doc was never done, so there's nothing to
do for that.

[1]: https://github.com/yamill/react-native-orientation#configuration

android/app/src/main/java/com/zulipmobile/MainActivity.java

@@ -1,7 +1,5 @@
 package com.zulipmobile;
 
-import android.content.Intent;
-import android.content.res.Configuration;
 import android.os.Bundle;
 import android.webkit.WebView;
 import com.facebook.react.ReactActivity;
@@ -22,12 +20,4 @@ protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         WebView.setWebContentsDebuggingEnabled(true);
     }
-
-    @Override
-    public void onConfigurationChanged(Configuration newConfig) {
-        super.onConfigurationChanged(newConfig);
-        Intent intent = new Intent("onConfigurationChanged");
-        intent.putExtra("newConfig", newConfig);
-        this.sendBroadcast(intent);
-    }
 }