crypto: hash - Remove cra_u.{digest,hash}

Remove unused digest_alg and hash_alg structs from crypto_alg union and
kill their definitions.  This also ensures that old-style digest/hash
algorithms maintained out of tree will break at build time rather than
oopsing at runtime.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>