crypto: virtio - remove dependency on CRYPTO_AUTHENC

virtio_crypto does not use function crypto_authenc_extractkeys, remove
this unnecessary dependency. Compiles fine and passes cryptodev-linux
cipher and speed tests from https://wiki.qemu.org/Features/VirtioCrypto

Fixes: dbaf0624

 ("crypto: add virtio-crypto driver")
Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>