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/notify-webhook_test.go

@@ -22,6 +22,7 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"path"
 	"testing"
 
@@ -51,7 +52,7 @@ func TestNewWebHookNotify(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer removeAll(root)
+	defer os.RemoveAll(root)
 
 	_, err = newWebhookNotify("1")
 	if err == nil {