Remove deprecated interfaces when advancing version to 1.0

A list of the items to remove:

* `HttpContentError`
* `HttpBinding#decode_rack_env`
* `HttpBinding#encode_structured_content`
* `HttpBinding#encode_batched_content`
* `HttpBinding#encode_binary_content`
* `:attributes` keyword argument for V0 and V1 event classes
* `deprecated_name` positional argument for `HttpBinding#register_formatter`