Remove /send_relation support

[bwindels]

As it has been removed from MSC 2674. The code in question is at

synapse/synapse/rest/client/v2_alpha/relations.py

Line 87 in 4b965c8

async def on_PUT_or_POST(

[clokep]

We should likely check if this is called at all -- are any clients using it?

[deepbluev7]

From what I know, neither any of the Elements, Nheko or Fluffy uses this. It might make sense to add logging to matrix.org for a few weeks to see if this is called and if nothing shows up, remove it. (I would be confused if Hydrogen would use this endpoint, considering who removed that endpoint from the MSC)

[richvdh]

I don't think this needs any extra info from the reporter

[clokep]

I was waiting for MSC2675 to merge to take a look at this, didn't realize that MSC2674 was what originally included this. I'll take a look!

[clokep]

This was specifically removed from the MSC in matrix-org/matrix-doc@db6213b (#2674) from MSC2674.