EVP_FETCH: remove the need to transport the legacy NID through construction

Now that the legacy NID isn't used as a main index for fetched
algorithms, the legacy NID was just transported around unnecessarily.
This is removed, and the legacy NID is simply set by EVP_{API}_fetch()
after the construction process is done.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8878)