Remove SSL_version_str

I was never exported in our shared libraries and no one noticed, and
we don't seem to use it ourselves, so clean it away.

In all likelyhood, this is a remain from the 90's, when it was in
fashion to litter library modules with these kinds of strings.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7340)