Remove debathena-build-depends

Per (Trac: #1462), debathena-build-depends is no longer needed and
we should stop depending on it.
Drop tex-config and dns-config; switch to dh7

- Switch to dh7 for building
- Drop conditional dependency on tex-config since all distros we
  support now have texlive 2009-10 (which honors /etc/papersize)
- Drop dependency on dns-config, which is becoming cluster-only