crypto: caam - remove unused sg_to_sec4_sg_len()

sg_to_sec4_sg_len() is no longer used since
commit 479bcc7c

 ("crypto: caam - Convert authenc to new AEAD interface")

Its functionality has been superseded by the usage of sg_nents_for_len()
returning the number of S/G entries corresponding to the provided length.

Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>