iOS: Delete FlutterPlatformViewsController.layerPoolSize (flutter/engine#56806)

This field is unused in the codebase/tests.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterPlatformViewsController.h

@@ -125,9 +125,6 @@
 
 - (size_t)embeddedViewCount;
 
-// TODO(cbracken): Delete. This is unused.
-- (size_t)layerPoolSize;
-
 // Returns the `FlutterPlatformView`'s `view` object associated with the view_id.
 //
 // If the `PlatformViewsController` does not contain any `FlutterPlatformView` object or

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterPlatformViewsController.mm

@@ -538,10 +538,6 @@ BOOL canApplyBlurBackdrop = YES;
   return self.compositionOrder.size();
 }
 
-- (size_t)layerPoolSize {
-  return self.layerPool->size();
-}
-
 - (UIView*)platformViewForId:(int64_t)viewId {
   return [self flutterTouchInterceptingViewForId:viewId].embeddedView;
 }