Remove unused apub audience field (fixes #5278)

[Nutomic]

This field doesnt serve any real purpose and can be removed.

[julianlam]

My understanding is audience is how Lemmy knows which community to slot new posts into?

Does Lemmy use to/cc instead?

[Nutomic]

Yes for posts it determines the community from to or cc fields. For comments it uses in_reply_to to get the associated post, and gets the community from there. And for votes and other actions, it gets the community from the associated post/comment.

[julianlam]

Thanks, this will simplify NodeBB code so it won't need to track the community.

I think there may be a use case for it down the line, though. Let me think on it a bit (though this doesn't affect the merging of this PR)

[julianlam]

One other note would be to amend FEP 1b12 with respect to this change 👍

[Nutomic]

The FEP is already finalized, so I dont think there is any possibility to change it (other than creating a new FEP which wouldnt make sense either).

[julianlam]

For comments it uses in_reply_to to get the associated post, and gets the community from there.

Reading this, it sounds like Lemmy traverses up the inReplyTo chain to determine community?

Semi-relatedly, this is where context would come in handy, as it could be used as a shortcut to the root node.

[dessalines]

Found a few more audience fields with grep:

Oops, accidentally clicked requested changes, meant to just comment

[silverpill]

Semi-relatedly, this is where context would come in handy, as it could be used as a shortcut to the root node.

@julianlam And context collection can have the same audience property, if desirable (I don't need it but other developers might find it useful).

Regarding 1b12, it doesn't require audience to be present, it only proposes it, so I think there is no need to update the FEP. A revised version of the FEP may be published, though (for example, it can describe backfilling and other new stuff).

[Nutomic]

@dessalines Removed it from the comment and test case. The other occurences in crates/apub/assets/ are examples of json data sent by other platforms so it should be unchanged.