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.