Fabric: clean up oldChildShadowView when handle Insert mount (#41155)

Summary:
We don't need to get oldChildShadowView when we handle the insert mount. So we can remove it.

## Changelog:

[IOS] [CHANGED] - Fabric: clean up oldChildShadowView when handling Insert mount

Pull Request resolved: https://github.com/facebook/react-native/pull/41155

Test Plan: None.

Reviewed By: sammy-SC

Differential Revision: D50554037

Pulled By: javache

fbshipit-source-id: 3250b6bbe119d800f05f39f790dc6949357d4f27