remove v1 send_join

[richvdh]

v1 send_join has been effectively deprecated since May 2020 (matrix-org/matrix-spec-proposals#2547).

Any servers still relying on it are likely to have other problems federating (any Synapse versions old enough not to support v2 are likely riddled with security holes).

Its presence in the spec is a source of confusion (cf #1514, #1515)

IMHO it's time we got rid of it. This will need an MSC though.