bhnd: remove srand() to ensure deterministic output

r310342 fixed non-deterministic nvram_map_gen.awk output and thus a non-
reproducible bhnd(4) build by using a unique sort key.

Go one step further and also remove the srand() call. There's no reason
we want non-deterministic behaviour from this script.

PR: 215422
Reported by: gjb (non-reproducibility of bhnd)
Reported by: lidl (srand as the cause)
Reviewed by: landonf
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8857