crypto: asymmetric_keys - Remove unused functions

encrypt_blob(), decrypt_blob() and create_signature() were some of the
functions added in 2018 by
commit 5a307718 ("KEYS: Provide missing asymmetric key subops for new
key type ops [ver #2]")
however, they've not been used.

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>