rpc: remove rate-limit middleware

[niklasad1]

Currently, we have added simple RPC rate-limit middleware and a couple of CLI flags to whitelist for "trusted peers" but it's not ideal to have this logic in the node itself because it could be handled by a separate proxy but most proxies works on the HTTP layer and not when connection has been upgraded to WebSocket connection. That's why it's currently deeply integrated in the node itself to limit individual RPC calls and not per connection basis.

https://github.com/AcalaNetwork/subway is very promising RPC proxy that support websocket connections and it provides much features such as caching, merging subscription and rate-limiting.

Once we have tested/some confidence that it works with rpc spec v2 impl then this code in substrate should be removed.

Originally posted by @niklasad1 in #3701 (comment)

/cc @PierreBesson @BulatSaif