crypto: hash - Remove legacy hash/digest code

6941c3a0 disabled compilation of the legacy digest code but didn't
actually remove it.  Rectify this.  Also, remove the crypto_hash_type
extern declaration from algapi.h now that the struct is gone.

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