backends/cryptodev: remove dead code

Fix Coverity issue 1390600.

Signed-off-by: Jay Zhou <jianjay.zhou@huawei.com>
Message-Id: <1524894864-7492-1-git-send-email-jianjay.zhou@huawei.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>