Get rid of roles/tarsnap/files/decrypted_tarsnap.key

[taoeffect]

Users should follow the tarsnap instructions and place the key in /root/.tarsnap/tarsnap.key. Having it look for the key in roles/tarsnap/files/decrypted_tarsnap.key is potentially dangerous (could get committed and pushed somewhere it's not supposed to), and it's just a weird location for it. I think we already did this with the SSL certs, so let's do it for tarsnap.

• Reference /root/.tarsnap/tarsnap.key in .tarsnaprc
• Update README.md as needed

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