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/object-api-multipart_test.go

@@ -19,6 +19,7 @@ package cmd
 import (
 	"bytes"
 	"fmt"
+	"os"
 	"strings"
 	"testing"
 
@@ -188,7 +189,7 @@ func testPutObjectPartDiskNotFound(obj ObjectLayer, instanceType string, disks [
 
 	// Remove some random disk.
 	for _, disk := range disks[:6] {
-		removeAll(disk)
+		os.RemoveAll(disk)
 	}
 
 	uploadIDs = append(uploadIDs, uploadID)
@@ -226,7 +227,7 @@ func testPutObjectPartDiskNotFound(obj ObjectLayer, instanceType string, disks [
 	// This causes quorum failure verify.
 	disks = disks[len(disks)-3:]
 	for _, disk := range disks {
-		removeAll(disk)
+		os.RemoveAll(disk)
 	}
 
 	// Object part upload should fail with quorum not available.