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/xl-v1-multipart_test.go

@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"os"
 	"testing"
 	"time"
 )
@@ -27,7 +28,7 @@ func TestUpdateUploadJSON(t *testing.T) {
 	if err != nil {
 		t.Fatalf("%s", err)
 	}
-	defer removeAll(root)
+	defer os.RemoveAll(root)
 
 	// Create an instance of xl backend
 	obj, fsDirs, err := prepareXL()