libnetwork/osl: remove dead code

The test-file had a duplicate definition for ErrNotImplemented, which
caused an error in this package, and was not used otherwise, so we can
remove this file.

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

libnetwork/osl/sandbox_unsupported_test.go

@@ -1,18 +0,0 @@
-//go:build !linux
-
-package osl
-
-import (
-	"errors"
-	"testing"
-)
-
-var ErrNotImplemented = errors.New("not implemented")
-
-func newKey(t *testing.T) (string, error) {
-	return "", ErrNotImplemented
-}
-
-func verifySandbox(t *testing.T, ns *Namespace) {
-	return
-}