Remove deprecation notice for binary_cache_secret_key_file

[Taneb]

It's still used for signing narinfo files if using Hydra as a binary cache server. See https://github.com/NixOS/hydra/blob/master/src/lib/Hydra/View/NARInfo.pm#L37

See also issues #548 and #679

[ivan]

Please merge this; the deprecation warning was very confusing to me as someone using a local store with store_uri = auto:

hydra-send-stats[3536553]: hydra.conf: binary_cache_secret_key_file is deprecated and ignored. use store_uri=...?secret-key= instead

1. It is not ignored with store_uri = auto and AFAIK binary_cache_secret_key_file is the only way have your packages be signed when they're downloaded over ssh:// (unless you choose to run nix store sign --all -k /var/secrets/nix/secret_key all the time).
2. store_uri=...?secret-key= does not work for auto as mentioned in https://github.com/NixOS/hydra/issues/679#issuecomment-532607341

Sorry, I probably tested wrong because my signing problem is still unsolved.