Desupporting -old and -b6 dump formats

[Greg Hudson]

While fleshing out our automated dump/load tests, I discovered that
loading a dump in -b6 or -old fails, and has failed since at least krb5
1.3.  I've written up the details in an RT issue[1].

Since this has been broken for a very long time and I haven't ever heard
anyone complain, I'd like to conclude that no one needs tools to migrate
off of a version of krb5 older than beta7, and we can just desupport the
-b6 and -old formats.  Are there any objections to doing this in 1.12?

[1] http://krbdev.mit.edu/rt/Ticket/Display.html?id=7563&user=guest&pass=guest