nanobsd: Stop copying ubldr

manu@ removed support for loading ubldr* from uboot last year. No need
to copy them to the image. This may be needed for some 32-bit platforms
in theory, but those platforms weren't ever the target for nanobsd that
I'm aware of. Should there be platforms where this is used, we can add
it to building those platforms.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D34191