Drop default boot-duration
    
    The file was last updated in 2008, which was kind of a while ago.
    Having files in /var conflicts with the rpm-ostree model of
    `/var` starting empty.
    
    Further, I don't think it makes sense to have a *global* boot
    duration - it should likely be per-Fedora Edition.  So probably
    we should delete the one in `/usr` as well and have the Edition
    packages carry it or something.