ios: remove needless method

We just call the parent class' method, so just get rid of it.

ios/app/src/ViewController.m

@@ -22,10 +22,6 @@
 
 @implementation ViewController
 
-- (void)didReceiveMemoryWarning {
-    [super didReceiveMemoryWarning];
-}
-
 - (void)viewDidLoad {
     [super viewDidLoad];