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/bucket-notification-handlers_test.go

@@ -25,6 +25,7 @@ import (
 	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"reflect"
 	"testing"
 )
@@ -51,7 +52,7 @@ func TestWriteNotification(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Unable to initialize test config %s", err)
 	}
-	defer removeAll(root)
+	defer os.RemoveAll(root)
 
 	var buffer bytes.Buffer
 	// Collection of test cases for each event writer.
@@ -116,7 +117,7 @@ func TestSendBucketNotification(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Unable to initialize test config %s", err)
 	}
-	defer removeAll(root)
+	defer os.RemoveAll(root)
 
 	eventCh := make(chan []NotificationEvent)