This removes some of the checks that were added in 0cba7740d4,
but should no longer be needed.
- `image/store.Delete()`: no need to validate image platform to delete it.
- `image/tarexporter/takeLayerReference()`: use `image.OperatingSystem()` and
fail early to prevent constructing the `ChainID()`.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>