refactor!(iroh-net): remove relay secret key (#2847)

The `secret_key` configuration of the relay server was actually dead
code at this point, this removes it from the config and all associated
dead code.
This also removes the need to write to the config file of the
`iroh-relay`, making deployments easier to secure.


## Breaking Changes

- removed 
  - the `secret_key` property of the `iroh-relay` config
  - `iroh_net::relay::server::ServerActorTask::secret_key`
  - `iroh_net::relay::server::ServerActorTask::public_key`
  - `iroh_net::relay::server::ServerActorTask::meta_cert`
  - field `secret_key` in `iroh_net::relay::server::RelayConfig`
- changed
- `iroh_net::relay::server::ServerActorTask::new` now takes no arguments

Closes #2845

Author: dignifiedquire