[video_player] Remove public access to the caption file

The existence of `Future? get closedCaptionFile` makes it impossible for the controller to transform the captions for its own use (e.g., sorting them for faster access) without extra complexity and memory; see https://github.com/flutter/packages/pull/8347 for context. There's no good reason I can see for the getter to be public; if the caller who set the caption file wants it for their own purposes, they can keep it directly instead of getting it from the player later.

Removing that public API at the next breaking change would let us do more optimizations to captions in the future without worrying about breaking API compat.