[pkg] Stop using /etc/bash_completion.d cubicweb-ctl shipped an empty /etc/bash_completion.d as a leftover of shipping a bash completion script. As pointed by Lintian, this directory is not obsolete, so let's get rid of it for good.

cubicweb source: source-is-missing cubicweb/web/data/jquery.flot.js line length is * characters (>512)