Remove mention of link between message digests and public key algorithms.

The comment in EVP_DigestInit.pod is:

> Returns the NID of the public key signing algorithm associated with this
digest. For example EVP_sha1() is associated with RSA so this will return
B<NID_sha1WithRSAEncryption>. Since digests and signature algorithms are no
longer linked this function is only retained for compatibility reasons.


I.e. there is no link anymore.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5682)