os: remove duplicate check from windows os.Stat

Fixes #21075

Change-Id: Idfe5002dfe17943844d9427e27f82ce894b92e80
Reviewed-on: https://go-review.googlesource.com/50270Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/stat_windows.go

@@ -16,9 +16,7 @@ func (file *File) Stat() (FileInfo, error) {
 	if file == nil {
 		return nil, ErrInvalid
 	}
-	if file == nil {
-		return nil, syscall.EINVAL
-	}
+
 	if file.isdir() {
 		// I don't know any better way to do that for directory
 		return Stat(file.dirinfo.path)