net/wifi-firmware-kmod: remove obsolete kernel module build code

All supported FreeBSD versions either shipped firmware along with
source and do not need this, or they support plain firmware file
loading.

Remove the now obsolete kernel module build code to simplify the
logic in the file. Further cleanup will be done once 13.5-R is
out of support in a few months.

Sponsored by: The FreeBSD Foundation
Reviewed by: jrm
Differential Revision: https://reviews.freebsd.org/D53679