Remove origin parameter from add_display_name_to_third_party_invite and add params to docstring (#6010)

Another small fixup noticed during work on a larger PR. The `origin` field of `add_display_name_to_third_party_invite` is not used and likely was just carried over from the `on_PUT` method of `FederationThirdPartyInviteExchangeServlet` which, like all other servlets, provides an `origin` argument.

Since it's not used anywhere in the handler function though, we should remove it from the function arguments.

synapse/federation/transport/server.py

@@ -575,7 +575,7 @@ class FederationThirdPartyInviteExchangeServlet(BaseFederationServlet):
 
     async def on_PUT(self, origin, content, query, room_id):
         content = await self.handler.on_exchange_third_party_invite_request(
-            origin, room_id, content
+            room_id, content
         )
         return 200, content