crypto_backend: remove superfluous memset fot hash and hmac contexts

The crypto backend crypt_hash ans crypt_hmac structs usually contain only pointers to internal crypto lib structures, no need to wipe them explicitly as there are no sensitive data. It is a crypto lib responsibility to remove sensitive data in destructor.

Only nettle backend directly contains hash context, keep it there.

This should also fix mysterious crashes in fuzzer with misaligned memset.