Get rid of `orig_obj` in `send` and `activitypub.postprocess_as2`

[snarfed]

It's kind of a mess, it's at least partially unnecessary, and it causes bugs like #1237.

One big catch is that most fediverse platforms don't support eg multiply-valued inReplyTo, so we still have to downgrade that - and other similar fields like object? - to singly valued for interop. https://funfedi.dev/support_tables/generated/in_reply_to/