[firebase_core] Deleting the default instance

[phamnhuvu-dev]

I read the document that it doesn't allow deleting the default instance
https://github.com/FirebaseExtended/flutterfire/blob/master/docs/core/usage.mdx#deleting-instances

But when I read the native library code, it allows deleting a default instance
https://github.com/firebase/firebase-ios-sdk/blob/master/FirebaseCore/Sources/FIRApp.m#L287-L312

Why don't we allow deleting a default instance on the dart library?