crypto: sm2 - Remove sm2 algorithm

The SM2 algorithm has a single user in the kernel.  However, it's
never been integrated properly with that user: asymmetric_keys.

The crux of the issue is that the way it computes its digest with
sm3 does not fit into the architecture of asymmetric_keys.  As no
solution has been proposed, remove this algorithm.

It can be resubmitted when it is integrated properly into the
asymmetric_keys subsystem.

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