Remove support for receiving unauthenticated SENDMEs.
We haven't generated these since Tor 0.3.5, which is no longer supported on the network.
Closes #914 (closed).
This branch is based on top of !1282 (merged); we shouldn't merge it till !1282 (merged) is in, so I'm marking it as Draft. Only the final commit in this branch is relevant to the review of this MR.