crypto: public_key: Remove redundant header file from public_key.h

The akcipher.h header file was originally introduced in SM2, and
then the definition of SM2 was moved to the existing code. This
header file is left and should be removed.

Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Ben Boeckel <mathstuf@gmail.com>