OpenSSH: remove ability to enable DSA support (in configure)

DSA is deprecated and upstream OpenSSH is in the process of removing it.

From OpenSSH-portable 10.0, cherry-pick the configure change so that we
can independently decide what to merge to stable branches.

This change has no direct user-facing impact, as the upstream configure
script is not used in the FreeBSD build process. It is worth noting as
part of OpenSSH's full removal of DSA support.

Reviewed by: jlduran, philip
Relnotes: Yes
Obtained from: OpenSSH-portable 6c9872faa1c2
Sponsored by: The FreeBSD Foundation