Retiring ssh client VersionAddendum

ssh supports a VersionAddendum configuration option to specify text to
append to the SSH version string. 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.  It is not supported upstream in the client.

The argument for supporting this in the client is not nearly as strong
as for the server, and I plan to retire this in the client 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.

I have a review open to make this change, https://reviews.freebsd.org/D32930.