Remove deprecated methods on Decoder

[fintelia]

Also removes current_chunk because it is no longer used elsewhere.

[HeroicKatora]

so I think that solves your concern?

Not really. My concern is that the replacement that the deprecations recommend don't appear to be (ergnomomic) replacements to me. I'd be much gladder about the outright removal if we had a better migration story for downstream consumers.

[HeroicKatora]

Not that we shouldn't do it regardless of migration, the breaking change does make sense because the current tracking of state isn't that well-designed either. In terms of maintenace, definitely better. I'd just be glad about a plan for replacing it.

[HeroicKatora]

Quoting relevant information from the discussion:

It is useless for the user to get a chunk of image data without knowing which one it is, so they have to be tracking that information anyway.

So, if anyone happens to run across this PR as the 'cause' of breakage then know that we are open to design ideas that reflect the above in a well-design interface.