Remove libkrb5* transitional packages from rkerberos

libkrb5-ruby* are part of ruby-krb5-auth in Debian main, and while rkerberos
is the successor to krb5-auth, claiming to replace the packages causes
conflicts as version numbers aren't equivalent.

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I'm trying to get the archives to pass dose-distcheck tests, this was flagged up.  The builds for squeeze/precise were done by @GregSutcliffe I think though, the files aren't in this repo.  They might need fixing manually...