Configure: cleanup @disable_cascade

'rsa', 'sha' and 'tlsext' can't be disabled, not even as a consequence
of other conditions, so having cascading disables that depend on them
is futile.  Clean up!

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4693)