daemon: remove leftover LCOW platform checks

This removes some of the checks that were added in 0cba7740d4,
but should no longer be needed.

- `Daemon.create()`: fix the error message, which assumed it could only occur on Windows.
- `Daemon.cleanupContainer()`: no need to validate container platform to delete it.
- `Daemon.containerExport`: if a container was created, we should be able to
  export it; no need to validate.

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

daemon/export.go

@@ -8,7 +8,6 @@ import (
 	"github.com/docker/docker/errdefs"
 	"github.com/docker/docker/pkg/archive"
 	"github.com/docker/docker/pkg/ioutils"
-	"github.com/docker/docker/pkg/system"
 )
 
 // ContainerExport writes the contents of the container to the given
@@ -47,9 +46,6 @@ func (daemon *Daemon) ContainerExport(name string, out io.Writer) error {
 }
 
 func (daemon *Daemon) containerExport(container *container.Container) (arch io.ReadCloser, err error) {
-	if !system.IsOSSupported(container.OS) {
-		return nil, fmt.Errorf("cannot export %s: %s ", container.ID, system.ErrNotSupportedOperatingSystem)
-	}
 	rwlayer, err := daemon.imageService.GetLayerByID(container.ID)
 	if err != nil {
 		return nil, err