cc@emmatyping@AA-Turner (can't request review)

Inpyzstd there was anEndlessZstdDecompressor class implemented in the C module. Some remains of the code were left, this PR aims to remove them.

In the comments,<D> refers toZstdDecompressor whereas<E> refers toEndlessZstdDecompressor. I cleaned that up as well.

There was_unused_char_for_align in theZstdDecompressor struct. My understanding is that it was placed only for optimization indecompressor_reset_session when setting several attributes to 0 at once. I removed it here, but if needed I can re-add it (twice to compensate the removal ofat_frame_edge).

• Issue:Implement PEP 784 - Adding Zstandard to the Python standard library #132983