os: fix another case where RemoveAll should return nil

This hopefully fixes issue 8793.

Fixes #8793

LGTM=iant
R=rsc, iant
CC=golang-codereviews
https://golang.org/cl/150860046

src/os/path.go

@@ -86,6 +86,9 @@ func RemoveAll(path string) error {
 	// Directory.
 	fd, err := Open(path)
 	if err != nil {
+		if IsNotExist(err) {
+			return nil
+		}
 		return err
 	}