Purge conffiles files deleted in version 1:1.90-1 (bug #645439)

The commit:

commit 75a48b26
Author: David Paleino <dapal@debian.org>
Date:   Thu Nov 3 12:42:39 2011 +0100

    Drop trigger-based completion loading

deleted the conffile /etc/cron.monthly/bash-completion, without updating
the maintainer scripts to purge the files.

This patch adds the files to debian/maintscript, setting the version to
the next release of bash-completion, as required by
dpkg-maintscript-helper:

  rm_conffile conffile [prior-version [package]]

  prior-version

    Defines the latest version of the package whose upgrade should
    trigger the operation. It is important to calculate prior-version
    correctly so that the operations are correctly performed even if the
    user rebuilt the package with a local version. If prior-version is
    empty or omitted, then the operation is tried on every upgrade
    (note: it's safer to give the version and have the operation tried
    only once).

    If the conffile has not been shipped for several versions, and you
    are now modifying the maintainer scripts to clean up the obsolete
    file, prior-version should be based on the version of the package
    that you are now preparing, not the first version of the package
    that lacked the conffile. This applies to all other actions in the
    same way.

    For example, for a conffile removed in version 2.0-1 of a package,
    prior-version should be set to 2.0-1~. This will cause the conffile
    to be removed even if the user rebuilt the previous version 1.0-1 as
    1.0-1local1. Or a package switching a path from a symlink (shipped
    in version 1.0-1) to a directory (shipped in version 2.0-1), but
    only performing the actual switch in the maintainer scripts in
    version 3.0-1, should set prior-version to 3.0-1~.