crypto: cavium - remove unused fucntions

cptvf_mbox_send_ack and cptvf_mbox_send_nack are never
used since introdution in commit c694b233 ("crypto: cavium
- Add the Virtual Function driver for CPT")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>