crypto: rmd256 - remove RIPE-MD 256 hash algorithm

RIPE-MD 256 is never referenced anywhere in the kernel, and unlikely
to be depended upon by userspace via AF_ALG. So let's remove it

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>