Remove individual user/community inboxes

[Nutomic]

Lemmy only uses https://example.com/inbox for federation, but also stores https://example.com/u/name/inbox for each user and community. This is completely useless, broken (https://github.com/LemmyNet/lemmy/issues/3928) and not required by the standard. So this PR removes the HTTP endpoints for /u/name/inbox, and only stores a single inbox url for each user/community.