Remove UIKit-oriented APIs

[defagos]

Description

This PR removes some APIs initially exposed for UIKit but which are not required anyway as a player should be implenented in SwiftUI, then wrapped into UIKit code if required.

Changes made

• Hide PictureInPicture.restoreFromInAppPictureInPicture().
• Hide PictureInPicture. enableInAppPictureInPictureWithCleanup(perform:).
• Update documentation to remove references to these APIs.

Checklist

• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).
• The playground has been updated (if relevant).