crypto: akcipher - Drop sign/verify operations

A sig_alg backend has just been introduced and all asymmetric
sign/verify algorithms have been migrated to it.

The sign/verify operations can thus be dropped from akcipher_alg.
It is now purely for asymmetric encrypt/decrypt.

Move struct crypto_akcipher_sync_data from internal.h to akcipher.c and
unexport crypto_akcipher_sync_{prep,post}():  They're no longer used by
sig.c but only locally in akcipher.c.

In crypto_akcipher_sync_{prep,post}(), drop various NULL pointer checks
for data->dst as they were only necessary for the verify operation.

In the crypto_sig_*() API calls, remove the forks that were necessary
while algorithms were converted from crypto_akcipher to crypto_sig
one by one.

In struct akcipher_testvec, remove the "params", "param_len" and "algo"
elements as they were only needed for the ecrdsa verify operation.
Remove corresponding dead code from test_akcipher_one() as well.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>