Revert container frame update change from #354 (#379)

Reverting that change as it turned out not to be necessary. At the moment of attaching screen we already have the frame set on the child VC. Updating it there again was cauinsg some visual glitches with old react-navigation screen container based implementation.

ios/RNSScreenContainer.m

@@ -54,6 +54,7 @@ - (void)insertReactSubview:(RNSScreenView *)subview atIndex:(NSInteger)atIndex
 {
   subview.reactSuperview = self;
   [_reactSubviews insertObject:subview atIndex:atIndex];
+  subview.frame = CGRectMake(0, 0, self.bounds.size.width, self.bounds.size.height);
 }
 
 - (void)removeReactSubview:(RNSScreenView *)subview
@@ -83,7 +84,9 @@ - (void)detachScreen:(RNSScreenView *)screen
 - (void)attachScreen:(RNSScreenView *)screen
 {
   [_controller addChildViewController:screen.controller];
-  screen.controller.view.frame = CGRectMake(0, 0, self.bounds.size.width, self.bounds.size.height);
+  // the frame is already set at this moment because we adjust it in insertReactSubview. We don't
+  // want to update it here as react-driven animations may already run and hence changing the frame
+  // would result in visual glitches
   [_controller.view addSubview:screen.controller.view];
   [screen.controller didMoveToParentViewController:_controller];
   [_activeScreens addObject:screen];