nanobsd: Stop copying ubldr

[imp]

Summary

manu@ removed support for loading ubldr* from uboot last year. No need
to copy them to the image.

Sponsored by: Netflix

[emaste]

Per IRC discussion ubldr is apparently still used on some ppc machines, but perhaps nanobsd is not relevant there.

[imp]

In D34191#773585, @emaste wrote:

Per IRC discussion ubldr is apparently still used on some ppc machines, but perhaps nanobsd is not relevant there.

I'm going to go ahead and commit this... If we need to add it back for some specific platform, we can do that once it's discovered this breaks a working system.