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/retry-storage_test.go

@@ -21,6 +21,7 @@ import (
 	"crypto/sha256"
 	"encoding/hex"
 	"errors"
+	"os"
 	"reflect"
 	"testing"
 	"time"
@@ -32,7 +33,7 @@ func TestRetryStorage(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer removeAll(root)
+	defer os.RemoveAll(root)
 
 	originalStorageDisks, disks := prepareXLStorageDisks(t)
 	defer removeRoots(disks)