arti: remove 'relay' subcommand and 'arti-relay' dependency

The 'arti-relay' package now builds a binary crate instead of a library crate. This means that it doesn't make sense to include it as a dependency of 'arti'. Also there are no immediate plans to run relays as an arti subcommand, so the boilerplate for that has been removed from 'arti'.

@Diziet I'm not able to replicate the error message that you see, but I think this should fix it. So I'm marking you as the reviewer on this.