mount: remove unused ErrNotImplementOnWindows

This error was added in c5843b7615, but no longer
used since a5a9f91832, which implemented Windows
support.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

mount/mount_windows.go

@@ -18,7 +18,6 @@ package mount
 
 import (
 	"encoding/json"
-	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -27,11 +26,6 @@ import (
 	"github.com/Microsoft/hcsshim"
 )
 
-var (
-	// ErrNotImplementOnWindows is returned when an action is not implemented for windows
-	ErrNotImplementOnWindows = errors.New("not implemented under windows")
-)
-
 // Mount to the provided target
 func (m *Mount) Mount(target string) error {
 	if m.Type != "windows-layer" {