syscall: remove unreachable code

syscall: remove unreachable code

Reported by go vet:

	$ go vet syscall
	# syscall_test
	# [syscall_test]
	syscall/env_unix_test.go:100:4: unreachable code

The TestVetStdlib test in golang.org/x/tools/go/analysis/unitchecker
also ran into this.

Fixes #73998.

Change-Id: I7f2842a42835a38163433a09a3311be9c30f8a14
Cq-Include-Trybots: luci.golang.try:x_tools-gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/702415
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>