xsalsa20poly1305: remove source code

[tarcieri]

The successor is the crypto_secretbox crate:

https://github.com/RustCrypto/nacl-compat/tree/master/crypto_secretbox

[jrose-signal]

If I look at the docs for crypto_secretbox it still suggests to use xsalsa20poly1305:

I could ask what changed, but that's probably not important, so I'll just stick with "is the current release up to date other than that"?

I misread "xsalsa20poly1305" as "xchacha20poly1305", no action needed. Though I'll note that deleting the changelog has added to the confusion here.

[tarcieri]

Yeah, xsalsa20poly1305 is retired, replaced by crypto_secretbox which supports both XSalsa20Poly1305 and libsodium-flavored XChaCha20Poly1305.

I could potentially add the CHANGELOG.md back but I'd really like to just completely delete the xsalsa20poly1305 directory at some point.