virtio: Silence a -Wunused warning

Remove virtio_swap_endian(). htole*() are nops on little-endian
platforms. No functional change intended.

MFC after: 1 week
Reviewed by: corvink, jhb
Differential Revision: https://reviews.freebsd.org/D37298

(cherry picked from commit db494ceb65ef301c8fb48f57b60c4f7a9cfebba5)