crypto: salsa20 - remove Salsa20 stream cipher algorithm

Salsa20 is not used anywhere in the kernel, is not suitable for disk
encryption, and widely considered to have been superseded by ChaCha20.
So let's remove it.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>