unix: move Unsetenv into env_unix.go

Unsetenv was in its own file for Go 1.4 compatibility. This version is
no longer supported, so move Unsetenv with the other environment
specific functions.

Change-Id: I81ed5182297598a7b3859711211ec8ef194bbe9a
Reviewed-on: https://go-review.googlesource.com/86538
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/env_unix.go

@@ -25,3 +25,7 @@ func Clearenv() {
 func Environ() []string {
 	return syscall.Environ()
 }
+
+func Unsetenv(key string) error {
+	return syscall.Unsetenv(key)
+}