ssh: retire client VersionAddendum

FreeBSD introduced VersionAddendum for the server as a local change in
2001 in commit 933ca70f8f88 and later extended it to the client in
commit 9e2cbe04ff4f.

In 2012 upstream added support for server VersionAddendum, in commit
23528816dc10. They do not support it for the client.

The argument for supporting this in the client is not nearly as strong
as for the server, so retire this option to reduce the scope of our
local patch set. This also avoids some cases of conflicts in ssh_config
during update, as a user's configuration would typically follow the
commented-out default VersionAddendum value.

Reviewed by: gordon, glebius
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32930