crypto: public_key: remove MPIs from public_key_signature struct

After digsig_asymmetric.c is converted the MPIs can be now
safely removed from the public_key_signature structure.

Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David Howells <dhowells@redhat.com>