[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.

etc/init.d
etc/cubicweb.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances