crypto: x86/cast5 - Remove unused cast5_ctr_16way

commit e2d60e2f ("crypto: x86/cast5 - drop CTR mode implementation")

removed the calls to cast5_ctr_16way but left the avx implementation.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>