[pkg] Tidy substvars usage in control file The documentation package was missing a Built-Using field to record the provenance of the templates. Empty packages or meta packages do not require Depends on a specific Python version, so we can remove the relevant substvars.

etc/init.d
etc/cubicweb.d
usr/bin
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances