IV generation is done only at AEAD level.
Support in ablkcipher is not needed, thus remove the dead code.

Link: https://www.mail-archive.com/search?l=mid&q=20160901101257.GA3362@gondor.apana.org.auSigned-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>