speed: make hmac(sha256) the default hmac

speed: make hmac(sha256) the default hmac

prefer hmac(sha256) rather than hmac(md5).  Also, drop the "skip_hmac"
label.  If we are supposed to do hmac(hash_func) and hash_func cannot
be found, then error out immediately.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22793)