How to remove native splash screen from flutter app?

That is not possible.

The native splash screen is a static image which is shown before any of the libraries in use for your app are loaded; it is shown before even the java libraries are finished loading.

The extra overhead of dart/flutter means that your application will take a little longer to load (particularly noticeable for iOS applications as an objc/swift app generally has less overhead than a Java one), so until it's all ready to go the only option is to show the native splash.

One approach you could use it to start with the native splash screen, and then have the 'flutter' one resemble it closely before animating to something else. But realistically, anything you use as a splash screen in flutter is most likely just going to be slowing down access to your app unless you have to do some sort of lengthy server communication every time the app starts.