crypto: shash - remove pointless checks of shash_alg::{export,import}

crypto_init_shash_ops_async() only gives the ahash tfm non-NULL
->export() and ->import() if the underlying shash alg has these
non-NULL.  This doesn't make sense because when an shash algorithm is
registered, shash_prepare_alg() sets a default ->export() and ->import()
if the implementor didn't provide them.  And elsewhere it's assumed that
all shash algs and ahash tfms have non-NULL ->export() and ->import().

Therefore, remove these unnecessary, always-true conditions.

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