Remove Windows' Readlink fork

This reverts the part of commit 8100e75 (PR containerd#113)
that added the fork of Readlink() for Windows.

At that time the fork was done to work around the bug
in golang's implementation of os.Readlink() for Windows.

The above bug was never reported upstream, but fortunately
it was independently found, reported [1], and fixed [2].
The fix made its way into go-1.13 (there is no mention of
that in release notes, so I checked it in git history).

[1] golang/go#30463
[2] https://go-review.googlesource.com/c/go/+/164201

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

driver/driver.go

@@ -138,6 +138,10 @@ func (d *driver) Lstat(p string) (os.FileInfo, error) {
 	return os.Lstat(p)
 }
 
+func (d *driver) Readlink(p string) (string, error) {
+	return os.Readlink(p)
+}
+
 func (d *driver) Mkdir(p string, mode os.FileMode) error {
 	return os.Mkdir(p, mode)
 }

driver/driver_unix.go

@@ -131,8 +131,3 @@ func (d *driver) LSetxattr(path string, attrMap map[string][]byte) error {
 func (d *driver) DeviceInfo(fi os.FileInfo) (maj uint64, min uint64, err error) {
 	return devices.DeviceInfo(fi)
 }
-
-// Readlink was forked on Windows to fix a Golang bug, use the "os" package here
-func (d *driver) Readlink(p string) (string, error) {
-	return os.Readlink(p)
-}

driver/driver_windows.go

@@ -18,8 +18,6 @@ package driver
 
 import (
 	"os"
-
-	"github.com/containerd/continuity/sysx"
 )
 
 func (d *driver) Mknod(path string, mode os.FileMode, major, minor int) error {
@@ -35,9 +33,3 @@ func (d *driver) Lchmod(path string, mode os.FileMode) (err error) {
 	// TODO: Use Window's equivalent
 	return os.Chmod(path, mode)
 }
-
-// Readlink is forked in order to support Volume paths which are used
-// in container layers.
-func (d *driver) Readlink(p string) (string, error) {
-	return sysx.Readlink(p)
-}