crypto: salsa20 - Revert "crypto: salsa20 - export generic helpers"

This reverts commit eb772f37, as now the
x86 Salsa20 implementation has been removed and the generic helpers are
no longer needed outside of salsa20_generic.c.

We could keep this just in case someone else wants to add a new
optimized Salsa20 implementation.  But given that we have ChaCha20 now
too, I think it's unlikely.  And this can always be reverted back.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>