Build: clean in po/ removes *~ files as well
    
    Editing work is done in Zanata UI so there is no point in keeping all
    versions around in SCM.
    
    https://fedorahosted.org/freeipa/ticket/6418
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Lukas Slebodnik <lslebodn@redhat.com>