Remove dconf profile, clean up maintscripts

- Do not ship a dconf profile, debathena-xsession will now provide
  it.
- Rename the dconf site database snippet files to include the
  package name, and not be first in the stack.
- Remove the hand-copied c-p-d snippets from the maintscripts
- Remove the hand-copied rm_conffile snippets and switch to
  dpkg-maintscript-helper and Pre-Depend on the appropriate
  version of dpkg