Remove outdated TODO

Old effort to complete the TODO rather than removing it:

Made get_hs_client_secret_keys_desc_enc_only function to abstract the secret key building based on a TODO that said the code was repetitive in the arti-client crate in the client module.

As the long function name suggests, it makes the secrets keys using only the desc_enc key and not the deprecated intro_auth, which may or may not be fine.

I also removed a (from my view) useless Result wrapping the return of the builder for the secret keys since it could never fail, though I feel like there might have been a reason for it to exist?