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/auth-handler_test.go

@@ -21,6 +21,7 @@ import (
 	"io"
 	"net/http"
 	"net/url"
+	"os"
 	"testing"
 )
 
@@ -324,7 +325,7 @@ func TestIsReqAuthenticated(t *testing.T) {
 	if err != nil {
 		t.Fatalf("unable initialize config file, %s", err)
 	}
-	defer removeAll(path)
+	defer os.RemoveAll(path)
 
 	creds, err := createCredential("myuser", "mypassword")
 	if err != nil {