Remove go1.9 specific code for windows (#5033)

Following fix https://go-review.googlesource.com/#/c/41834/ has
been merged upstream and released with go1.9.

cmd/posix-utils_windows.go

@@ -19,17 +19,9 @@
 package cmd
 
 import (
-	"os"
 	"strings"
-
-	os2 "github.com/minio/minio/pkg/x/os"
 )
 
-// Wrapper around safe stat implementation to avoid windows bugs.
-func osStat(name string) (os.FileInfo, error) {
-	return os2.Stat(name)
-}
-
 // isValidVolname verifies a volname name in accordance with object
 // layer requirements.
 func isValidVolname(volname string) bool {