Don't maintain URLs for foreign layers when pushing them

[drcapulet]

For images with non-distributable layers, if we're pushing the layer to the destination we want to remove any URLs on the layer since they're generally preferred over the repository itself (containerd example) - this is important for deployments to air-gapped installations that don't have access to any of the URLs.

[drcapulet]

@mtrmac Appreciate the review - funny enough the day I opened this Microsoft dropped the usage of foreign layers, I'm inclined to just close this out?

I can't personally tell on the expected behavior for URLs from the spec.

[mtrmac]

I do think you’ve touched on a feature gap and/or a bug, and it’s worth fixing in principle.

OTOH the real-world need for the foreign layer feature is really becoming much less urgent. If you want to close this PR, I think that’s completely fair.