The randomTrap const is initialized to a non-zero value for linux in
getrandom_linux_$GOARCH.go and for freebsd in getrandom_freebsd.go
directly since CL 16662. Thus, omit the unnecessary check.

Change-Id: Id20cd628dfe6fab9908fa5258c3132e3b422a6b4
Reviewed-on: https://go-review.googlesource.com/c/144108
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>