[test] remove custom implementation of FakeRequest.header_accept_language() (closes #3375021) ``Accept-Language`` should be overridable just as any other request header. Default value is "nothing" since the application should be able the empty header case.

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