geom_label: Remove an old sysinstall(8) workaround

[jrtc27]

Summary

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).

[imp]

I think this is OK, but you might want to add kirk to the reviewers. He knows about weird setups like this...

[mckusick]

Basing a decision on just the superblock magic number seems a bit weak, but that is what has been used traditionally and it worked, so with that in mind, this change seems reasonable.
Were this still in use, I would recommend more robust checks. For example verifying the check-hash for UFS2 filesystems that have them.