Remove FlutterActivity#onFlutterReady() (flutter/engine#3806)

Fixes https://github.com/flutter/flutter/issues/10571

engine/src/flutter/shell/platform/android/io/flutter/app/FlutterActivity.java

@@ -35,15 +35,6 @@ public class FlutterActivity extends Activity implements FlutterView.Provider, P
         return viewProvider.getFlutterView();
     }
 
-    /**
-     * Hook for subclasses to customize their startup behavior.
-     *
-     * @deprecated Just override {@link #onCreate(Bundle)} instead, and add your
-     * logic after calling {@code super.onCreate()}.
-     */
-    @Deprecated
-    protected void onFlutterReady() {}
-
     /**
      * Hook for subclasses to customize the creation of the
      * {@code FlutterView}.
@@ -75,7 +66,6 @@ public class FlutterActivity extends Activity implements FlutterView.Provider, P
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         eventDelegate.onCreate(savedInstanceState);
-        onFlutterReady();
     }
 
     @Override