crypto: atmel_sha - remove unused shash fallback instance.

The fallback is never used, so there is no point in having it.

The cra_exit routine can also be removed, since all it did was releasing
the fallback, along with the stub around cra_init, which just added an
unused NULL argument.

Signed-off-by: Svenning Soerensen <sss@secomea.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>