posix: Deprecate custom removeAll/mkdirAll implementations. (#4808)

Since go1.8 os.RemoveAll and os.MkdirAll both support long
path names i.e UNC path on windows. The code we are carrying
was directly borrowed from `pkg/os` package and doesn't need
to be in our repo anymore. As a side affect this also
addresses our codecoverage issue.

Refer #4658

cmd/posix-list-dir-others.go

@@ -27,7 +27,7 @@ import (
 
 // Return all the entries at the directory dirPath.
 func readDir(dirPath string) (entries []string, err error) {
-	d, err := os.Open(preparePath(dirPath))
+	d, err := os.Open((dirPath))
 	if err != nil {
 		// File is really not found.
 		if os.IsNotExist(err) {
@@ -55,7 +55,7 @@ func readDir(dirPath string) (entries []string, err error) {
 			// Stat symbolic link and follow to get the final value.
 			if fi.Mode()&os.ModeSymlink == os.ModeSymlink {
 				var st os.FileInfo
-				st, err = osStat(preparePath(path.Join(dirPath, fi.Name())))
+				st, err = osStat((path.Join(dirPath, fi.Name())))
 				if err != nil {
 					errorIf(err, "Unable to stat path %s", path.Join(dirPath, fi.Name()))
 					continue