libct/user: rm windows code

Commit bf74951 added these two functions, but they are only used from
Windows code. The v1 of this patch moved these functions to _windows.go
file, but after some discussion we decided to drop windows code
altogether, so this is what this patch now does.

This fixes

> libcontainer/user/user.go:64:6: func `groupFromOS` is unused (unused)
> libcontainer/user/user.go:35:6: func `userFromOS` is unused (unused)

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

libcontainer/user/user.go

@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"os/user"
 	"strconv"
 	"strings"
 )
@@ -29,52 +28,13 @@ type User struct {
 	Shell string
 }
 
-// userFromOS converts an os/user.(*User) to local User
-//
-// (This does not include Pass, Shell or Gecos)
-func userFromOS(u *user.User) (User, error) {
-	newUser := User{
-		Name: u.Username,
-		Home: u.HomeDir,
-	}
-	id, err := strconv.Atoi(u.Uid)
-	if err != nil {
-		return newUser, err
-	}
-	newUser.Uid = id
-
-	id, err = strconv.Atoi(u.Gid)
-	if err != nil {
-		return newUser, err
-	}
-	newUser.Gid = id
-	return newUser, nil
-}
-
 type Group struct {
 	Name string
 	Pass string
 	Gid  int
 	List []string
 }
 
-// groupFromOS converts an os/user.(*Group) to local Group
-//
-// (This does not include Pass or List)
-func groupFromOS(g *user.Group) (Group, error) {
-	newGroup := Group{
-		Name: g.Name,
-	}
-
-	id, err := strconv.Atoi(g.Gid)
-	if err != nil {
-		return newGroup, err
-	}
-	newGroup.Gid = id
-
-	return newGroup, nil
-}
-
 // SubID represents an entry in /etc/sub{u,g}id
 type SubID struct {
 	Name  string