geom_label: Remove an old sysinstall(8) workaround

We removed sysinstall(8) back in 2011, so this workaround should be long
since unnecessary. This workaround can end up breaking cases that are
hit in the real world, such as dd'ing a small pre-built disk image to a
large partition that you intend to grow on first boot and uses a UFS
disk label for / in its /etc/fstab (as the only reliable thing a raw UFS
image can reference).

Reviewed by: imp, mckusick
Differential Revision: https://reviews.freebsd.org/D30825