Cleanup enableVirtualViewDebugFeatures and FlingItemOverlay (#56946)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56946

The `enableVirtualViewDebugFeatures` feature flag was effectively dead — its only MobileConfig backing param (`rn_fling.fling_debug`) hadn't been updated in 365+ days, and the codemod-tracking diff D105936405 was already queued to remove the three native overrides. Rather than carry the FF + JS overlay surface forward, delete the whole stack: the FF, the overrides, the BUCK dep, the JS overlay components (`FlingItemOverlay`, `FlingDebugItemOverlay`, `FlingDebugOverlay`), the debug-log helpers in the native ScrollView/VirtualView implementations on both iOS and Android, and the now-orphaned references in `VirtualCollectionView`.

Net: 4 files deleted, the FF removed from the cross-language codegen, native debug logging dropped from 5 files (1 ObjC++, 4 Kotlin), and consumer code in `VirtualCollectionView` simplified.

Changelog:
[General][Removed] - Remove unused `enableVirtualViewDebugFeatures` feature flag and the associated `FlingItemOverlay` / `FlingDebugItemOverlay` debug surfaces.

Reviewed By: lunaleaps

Differential Revision: D105959433

fbshipit-source-id: 40cba737f3191f3e2501677a16b8b738ec9310c5